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238 Cards in this Set
- Front
- Back
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calat-
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something inserted
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erg-
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work
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-gram
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something written
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hyper-
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over, more
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inter-
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between
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iso-
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equal
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laten-
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hidden
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syn-
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together
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troposphere
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*where weather occurs
*first layer of atmosphere *thickest layer of atmosphere |
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mesophere
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*coldest layer of the atmosphere
*meteors burn up in this layer |
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stratosphere
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*second layer of the atmosphere
*protects planet from UV radiation *ozone |
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ABSOLUTE HUMIDITY
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*A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.
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ABSOLUTE INSTABILITY
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*When the lapse rate of a column of air is greater than the dry adiabatic lapse rate. The term absolute is used because this applies whether or not the air is dry or saturated.
Related term: instability |
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ABSOLUTE TEMPERATURE SCALE
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*A temperature scale with a freezing point of +273°K (Kelvin) and a boiling point of +373°K.
Related term: Kelvin Temperature Scale |
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ABSOLUTE ZERO
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Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0° Kelvin, -273.15° Celsius and -459.67° Fahrenheit.
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ABSORPTION
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The process in which incident radiant energy is retained by a substance. The absorbed radiation is then transformed into molecular energy.
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BACKING
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A counterclockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is clockwise. This can either happen in the horizontal or the vertical (with height). For example, the wind shifts from the northeast to the north to the northwest. It is the opposite of veering.
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BACKSCATTER
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A radar echo that is reflected, or scattered, at 180 degrees to the direction of the incident wave. Also the scattering of radiant energy into space before it reaches the earth's surface.
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BALL LIGHTNING
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A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air.
Related term: globe lightning |
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BAROCLINITY
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The state of stratification in a fluid in which surfaces of constant pressure intersect surfaces of constant density. Also known as baroclinicity. An example is the tight temperature gradient along the East Coast of the United States during the winter that gives rise to intense cyclogenesis.
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BAROGRAPH
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An instrument that continuously records a barometer's reading of atmospheric pressure.
Related term: aneroid barometer |
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ABYSSAL PLAIN
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*The flat, gently sloping or nearly level region of the sea floor.
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ADIABATIC PROCESS
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*A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In this process, compression will result in warming and expansion will result in cooling.
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ADVECTION
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*The horizontal transfer of any property in the atmosphere by the movement of air (wind). Examples include heat and moisture advection.
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ADVECTION FOG
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*Fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point. Related terms: Arctic Sea Smoke and sea fog
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ADVISORY
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*Statements that are issued by the National Weather Service for probable weather situations of inconvenience that do not carry the danger of warning criteria, but, if not observed, could lead to hazardous situations. Some examples include snow advisories stating possible slick streets, or fog advisories for patchy fog condition causing temporary restrictions to visibility.
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BAROMETER
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An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer.
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BAROMETRIC PRESSURE
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The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).
Related term: atmospheric pressure |
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BAROTROPY
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The state of a fluid in which surfaces of constant density or temperature are coincident with surfaces of constant pressure. It is considered zero baroclinity.
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BATHYTHERMOGRAPH
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A device used to obtain a record of temperature against depth (pressure) in the ocean. May be referred to as a B.T.
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BEAUFORT WIND SCALE
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Refers to the winds in the Canadian Arctic that blow through the narrow Bellot Strait between Somerset Island and the Boothia Peninsula, connecting the Gulf of Boothia and Franklin Strait.
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AFOS
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*Acronym for Automation of Field Operations and Services. It is the computer system that links National Weather Service offices together for weather data transmission.
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AIR
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*This is considered the mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are Nitrogen (N2) at 78.09%, Oxygen (O2) at 20.946%, Argon (A) at 0.93%, and Carbon Dioxide (CO2) at 0.033%. One of the most important constituents of air and most important gases in meteorology is water vapor (H2O).
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AIR MASS
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*An extensive body of air throughout which the horizontal temperature and moisture characteristics are similar.
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AIR MASS THUNDERSTORM
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*A thunderstorm that is produced by convection within an unstable air mass through an instability mechanism. Such thunderstorms normally occur within a tropical or warm, moist air mass during the summer afternoon as the result of afternoon heating and dissipate soon after sunset. Such thunderstorms are not generally associated with fronts and are less likely to become severe than other types of thunderstorms. However, that does not preclude them from having brief heavy downpours.
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AIR POLLUTION
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*The soiling of the atmosphere by contaminants to the point that may cause injury to health, property, plant, or animal life, or prevent the use and enjoyment of the outdoors
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BERMUDA HIGH
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A statement of the conservation of energy for a steady, nonviscous, incompressible level flow. It is an inverse relationship in which pressures are least where velocities are greatest. Theorized by Daniel Bernoulli (1700-1782), a Swiss mathematician and physicist.
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BIOSPHERE
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The transition zone between the earth and the atmosphere within which most terrestrial life forms are found. It is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere.
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BLACK BLIZZARD
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A local term for a violent duststorm on the south-central Great Plains that darkens the sky and casts a pall over the land.
Related term: black roller |
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BLACK ICE
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Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces.
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BLIZZARD
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A severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficient falling and/or blowing snow in the air to frequently reduce visibility to 1/4 mile or less for a duration of at least 3 hours. A severe blizzard is characterized by temperatures near or below 10°F, winds exceeding 45 mph, and visibility reduced by snow to near zero.
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BLOCKING HIGH
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The development of a warm ridge or cutoff high aloft at high latitudes which becomes associated with a cold high at the surface, causing a split in the westerly winds. Such a high will move very slowly, tending to move westward during intensification and eastward during dissipation. It prevents the movement of migratory cyclones across its latitudes.
Related terms: cut-off high and Omega block |
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AIR QUALITY STANDARDS
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*The maximum level which will be permitted for a given pollutant. Primary standards are to be sufficiently stringent to protect the public health. Secondary standards must protect the public welfare, including property and aesthetics.
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ALASKAN WINDS
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*The downslope air flow that blows through the Alaskan valleys. It is usually given local names, such as Knik, Matanuska, Pruga, Stikine, Taku, Take, Turnagain, or Williwaw.
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ALBEDO
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*The ratio of the amount of radiation reflected from an object's surface compared to the amount that strikes it. This varies according to the texture, color, and expanse of the object's surface and is reported in percentage. Surfaces with high albedo include sand and snow, while low albedo rates include forests and freshly turned earth.
Related term: Dave's Dictionary |
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BLOWING DUST
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Dust that is raised by the wind to heights of six feet or greater. It is reported as "BLDU" in an observation and on the METAR.
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BLOWING SAND
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Sand that is raised by the wind to heights of six feet or greater. It is reported as "BLSA" in an observation and on the METAR.
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BLOWING SNOW
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Snow that is raised by the wind to heights of six feet or greater. It is reported as "BLSN" in an observation and on the METAR.
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BLOWING SPRAY
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Salt spray that is raised by the wind to heights of six feet or greater. It is reported as "BLPY" in an observation and on the METAR.
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BLUE NORTHER
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Refers to a swift-moving cold frontal passage in the southern Great Plains, marked by a dark, blue-black sky with strong wintery winds from the northwest or north and temperatures that may drop 20°F to 30°F in a few minutes.
Related term: Texas Norther |
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BOILING POINT
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The temperature at which a liquid changes to a vaporous state. The temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. The boiling point of pure water at standard pressure is 100°C or 212°F.
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BOULDER WIND
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A local name referring to an extremely strong downslope wind in the Front Range of the Rocky Mountains near Boulder, Colorado.
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BOUNDARY LAYER
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The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or one kilometer, from the earth's surface, where the wind is influenced by the friction of the earth's surface and the objects on it.
Related terms: surface boundary layer and friction layer |
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BOW ECHO
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A radar echo signature often associated with severe thunderstorms, especially those that produce wind damage. It is bent outward in a "bow" shape.
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BOYLE'S LAW
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States that when the temperature is held constant, the volume of a gas is inversely proportional to its pressure. Therefore, if the pressure increases, the volume decreases and visa versa. For example, if the volume if halved, then the pressure is doubled. If the temperature is held constant, it becomes an isothermal process. Discovered by Robert Boyle (1627-1691), an Irish physicist and chemist and co-founder of the Royal Society.
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BRIGHT BAND
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A narrow, intense radar echo due to water-covered ice particles at the melting level where reflectivity is at its greatest.
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BROKEN
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The amount of sky cover for a cloud layer between 5/8ths and 7/8ths, based on the summation layer amount for that layer.
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BUBBLE HIGH
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A small high that may be created by precipitation and vertical instability associated with thunderstorm activity. A product of downdrafts, it is relatively cold and often has the characteristics of a different air mass. Convergence along the leading edge of a bubble high may help form additional thunderstorms.
Related term: meso high |
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BUYS BALLOT'S LAW
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Describes the relationship of the horizontal wind direction to the pressure distribution. In the Northern Hemisphere, if one stands with one's back to the wind, the pressure on one's left is lower than the pressure on one's right. It is reversed in the Southern Hemisphere. This law was named after the Dutch meteorologist, Buys Ballot, who developed the formula in 1857.
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BWER
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Acronym for Bounded Weak Echo Region. Refers to radar echo signatures with low reflectivity in the center, surrounded by higher reflectivity. It is associated with strong updrafts and is found in the inflow region of a thunderstorm.
Related term: vault |
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ALBERTA CLIPPER
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A fast moving, snow-producing weather system that originates in the lee of the Canadian Rockies. It moves quickly across the northern United States, often bring gusty winds and cold Arctic air.
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ALEUTIAN LOW
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A semi-permanent, subpolar area of low pressure located in the Gulf of Alaska near the Aleutian Islands. It is a generating area for storms and migratory lows often reach maximum intensity in this area. It is most active during the late fall to late spring. During the summer, it is weaker, retreating towards the North Pole and becoming almost nonexistent. During this time, the North Pacific High pressure system dominates.
Related term: Icelandic Low |
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ALTIMETER
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An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure.
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ALTIMETER SETTING
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The pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude above mean sea level of an aircraft on the ground at the location for which the value was determined
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ALTITUDE
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In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.
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ALTOCUMULUS
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Composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of water droplets. In the mid-latitudes, cloud bases are usually found between 8,000 and 18,000 feet. A defining characteristic is that it often appears as a wavy billowy layer of cloud, giving it the nickname of "sheep" or "woolpack" clouds. Sometimes confused with cirrocumulus clouds, its elements (individual clouds) have a larger mass and cast a shadow on other elements. It may form several sub-types, such as altocumulus castellanus or altocumulus lenticularis. Virga may also fall from these clouds.
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ALTOCUMULUS CASTELLANUS
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A middle cloud with vertical development that forms from altocumulus clouds. It is composed primarily of ice crystals in its higher portions and characterized by its turrets, protuberances, or crenelated tops. Its formation indicates instability and turbulence at the altitudes of occurrence.
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ALTOSTRATUS
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This middle cloud genus is composed of water droplets, and sometimes ice crystals, In the mid-latitudes, cloud bases are generally found between 15,000 and 20,000 feet. White to gray in color, it can create a fibrous veil or sheet, sometimes obscuring the sun or moon. It is a good indicator of precipitation, as it often precedes a storm system. Virga often falls from these clouds.
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AMERICAN METEOROLOGICAL SOCIETY
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An organization whose membership promotes the education and professional advancement of the atmospheric, hydrologic, and oceanographic sciences.
For further information, contact the AMS. Related term: National Weather Association |
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ANABATIC WIND
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A wind that is created by air flowing uphill. Valley breezes, produced by local daytime heating, are an example of these winds. The opposite of a katabatic wind.
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ANEMOMETER
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An instrument that measures the speed or force of the wind.
Related term: Dave's Dictionary |
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ANEROID BAROMETER
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An instrument for measuring the atmospheric pressure. It registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. The scales on the glass cover measure pressure in both inches and millibars.
Related term: mercurial barometer |
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ANOMALOUS PROPAGATION
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This refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. May be referred to as A.P.
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ANTARCTIC
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Of or relating to the area around the geographic South Pole, from 90° South to the Antarctic Circle at approximately 66 1/2°South latitude, including the continent of Antarctica. Along the Antarctic Circle, the sun does not set on the day of the summer solstice (approximately December 21st) and does not rise on the day of the winter solstice (approximately June 21st).
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ANTARCTIC OCEAN
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Although not officially recognized as a separate ocean body, it is commonly applied to those portions of the Atlantic, Pacific, and Indian Oceans that reach the Antarctic continent on their southern extremes.
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ANEMOMETER
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An instrument that measures the speed or force of the wind.
Related term: Dave's Dictionary |
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ANEROID BAROMETER
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An instrument for measuring the atmospheric pressure. It registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. The scales on the glass cover measure pressure in both inches and millibars.
Related term: mercurial barometer |
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ANOMALOUS PROPAGATION
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This refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. May be referred to as A.P.
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ANTARCTIC
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Of or relating to the area around the geographic South Pole, from 90° South to the Antarctic Circle at approximately 66 1/2°South latitude, including the continent of Antarctica. Along the Antarctic Circle, the sun does not set on the day of the summer solstice (approximately December 21st) and does not rise on the day of the winter solstice (approximately June 21st).
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ANTARCTIC OCEAN
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Although not officially recognized as a separate ocean body, it is commonly applied to those portions of the Atlantic, Pacific, and Indian Oceans that reach the Antarctic continent on their southern extremes.
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ALTOCUMULUS
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Composed of flattened, thick, gray, globular masses, this middle cloud genus is primarily made of water droplets. In the mid-latitudes, cloud bases are usually found between 8,000 and 18,000 feet. A defining characteristic is that it often appears as a wavy billowy layer of cloud, giving it the nickname of "sheep" or "woolpack" clouds. Sometimes confused with cirrocumulus clouds, its elements (individual clouds) have a larger mass and cast a shadow on other elements. It may form several sub-types, such as altocumulus castellanus or altocumulus lenticularis. Virga may also fall from these clouds.
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ALTOCUMULUS CASTELLANUS
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A middle cloud with vertical development that forms from altocumulus clouds. It is composed primarily of ice crystals in its higher portions and characterized by its turrets, protuberances, or crenelated tops. Its formation indicates instability and turbulence at the altitudes of occurrence.
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ALTOSTRATUS
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This middle cloud genus is composed of water droplets, and sometimes ice crystals, In the mid-latitudes, cloud bases are generally found between 15,000 and 20,000 feet. White to gray in color, it can create a fibrous veil or sheet, sometimes obscuring the sun or moon. It is a good indicator of precipitation, as it often precedes a storm system. Virga often falls from these clouds.
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AMERICAN METEOROLOGICAL SOCIETY
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An organization whose membership promotes the education and professional advancement of the atmospheric, hydrologic, and oceanographic sciences.
For further information, contact the AMS. Related term: National Weather Association |
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ANABATIC WIND
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A wind that is created by air flowing uphill. Valley breezes, produced by local daytime heating, are an example of these winds. The opposite of a katabatic wind.
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ANEMOMETER
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An instrument that measures the speed or force of the wind.
Related term: Dave's Dictionary |
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ANEROID BAROMETER
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An instrument for measuring the atmospheric pressure. It registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure. The aneroid is a thin-walled metal capsule or cell, usually made of phosphor bronze or beryllium copper. The scales on the glass cover measure pressure in both inches and millibars.
Related term: mercurial barometer |
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ANOMALOUS PROPAGATION
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This refers to the non-standard propagation of a beam of energy, radio or radar, under certain atmospheric conditions, appearing as false (non-precipitation) echoes. May be referred to as A.P.
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ANTARCTIC
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Of or relating to the area around the geographic South Pole, from 90° South to the Antarctic Circle at approximately 66 1/2°South latitude, including the continent of Antarctica. Along the Antarctic Circle, the sun does not set on the day of the summer solstice (approximately December 21st) and does not rise on the day of the winter solstice (approximately June 21st).
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ANTARCTIC OCEAN
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ANTICYCLONE
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ANTICYCLONE
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A relative pressure maximum. An area of pressure that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. It is the opposite of an area of low pressure, or a cyclone.
Related term: high pressure |
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ANVIL
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The upper portion of a cumulonimbus cloud that becomes flat and spread-out, sometimes for hundreds of miles downstream from the parent cloud. It may look smooth or fibrous, but in shape, it resembles a blacksmith's anvil. It indicates the mature or decaying stage of a thunderstorm.
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APHELION
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The point on the earth's orbit that is farthest from the sun. Although the position is part of a 21,000 year cycle, currently it occurs around July, when the earth is about 3 million miles farther from the sun than at perihelion. This term can be applied to any other celestial body in orbit around the sun. It is the opposite of perihelion.
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APOGEE
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The point farthest from the earth on the moon's orbit. This term can be applied to any other body orbiting the earth, such as satellites. It is the opposite of perigee.
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ARCTIC
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Of or relating to the area around the geographic North Pole, from 90° North to the Arctic Circle at approximately 66 1/2 North latitude.
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ARCTIC AIR MASS
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An air mass that develops around the Arctic, it is characterized by being cold from surface to great heights. The boundary of this air mass is often defined by the Arctic front, a semi-permanent, semi-continuous feature. When this air mass moves from its source region, it may become more shallow in height as it spreads southward.
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ARCTIC JET
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The jet stream that is situated high in the stratosphere in and around the Arctic or Antarctic Circles. It marks the boundary of polar and arctic air masses.
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ARCTIC SEA SMOKE
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A type of advection fog that forms primarily over water when cold air passes across warmer waters.
Related term: steam fog |
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ARGON (A)
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A colorless, odorless inert gas that is the third most abundant constituent of dry air, comprising 0.93% of the total.
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ARID
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A term used for an extremely dry climate. The degree to which a climate lacks effective, life-promoting moisture. It is considered the opposite of humid when speaking of climates.
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ASOS
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Acronym for Automated Surface Observing System. This system is a collection of automated weather instruments that collect data. It performs surface based observations from places that do not have a human observer, or that do not have an observer 24 hours a day.
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ASTRONOMICAL TWILIGHT
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The time after nautical twilight has commenced and when the sky is dark enough, away from the sun's location, to allow astronomical work to proceed. It ends when the center of the sun is 18° below the horizon.
Related term: twilight |
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ATMOSPHERE
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The gaseous or air portion of the physical environment that encircles a planet. In the case of the earth, it is held more or less near the surface by the earth's gravitational attraction. The divisions of the atmosphere include the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere.
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ATMOSPHERIC PRESSURE
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The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).
Related term: barometric pressure |
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AURORA
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It is created by the radiant energy emission from the sun and its interaction with the earth's upper atmosphere over the middle and high latitudes. It is seen as a bright display of constantly changing light near the magnetic poles of each hemisphere. In the Northern Hemisphere, it is known as the aurora borealis or Northern Lights, and in the Southern Hemisphere, this phenomena is called the aurora australis.
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AUTUMN
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The season of the year which occurs as the sun approaches the winter solstice, and characterized by decreasing temperatures in the mid-latitudes. Customarily, this refers to the months of September, October, and November in the North Hemisphere and the months of March, April, and May in the Southern Hemisphere. Astronomically, this is the period between the autumnal equinox and the winter solstice.
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AVHRR
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Acronym for Advanced Very High Resolution Radiometer. It is the main sensor on the U.S. polar orbiting satellites.
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AVIATION WEATHER CENTER
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As one of the National Centers for Environmental Prediction, it is the national center for weather information that is used daily by the Federal Aviation Administration, commercial airlines, and private pilots. It is entering a new phase of service, growing to accept global forecasting responsibilities.
For further information, contact the AWC, located in Kansas City, Missour |
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AWIPS
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Acronym for Advanced Weather Interactive Processing System. It is the computerized system that processes NEXRAD and ASOS data received at National Weather Service Forecast Offices.
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AZORES HIGH
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A semi-permanent, subtropical area of high pressure in the North Atlantic Ocean that migrates east and west with varying central pressure. Depending on the season, it has different names. In the Northern Hemispheric winter and early spring, when the Icelandic Low dominates the North Atlantic, it is primarily centered near the Azores Islands. When it is displaced westward, during the summer and fall, the center is located in the western North Atlantic, near Bermuda, and is known as the Bermuda High.
Related term: North Pacific High |
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GALE
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On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological Organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots).
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GALE WARNING
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A warning for marine interests for impending winds from 28 to 47 knots (32 to 54 miles per hour).
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GEOPHYSICS
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The study of the physics or nature of the Earth and its environment. It deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. Areas of studies include the atmospheric sciences and meteorology, geology, seismology, and volcanology, and oceanography and related marine sciences, such as hydrology. By extension, it often includes astronomy and the related astro-sciences.
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GEOSPHERE
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Considered the solid portions of the earth, including the hydrosphere and the lithosphere, as opposed to the atmosphere, which lies above it. At their conjunction is the biosphere.
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GEOSTATIONARY SATELLITE
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An orbiting weather satellite that maintains the same position over the equator during the earth's rotation. Also known as GOES, an acronym for Geostationary Operational Environmental Satellite. Related terms: polar-orbiting satellite and Dave's Dictionary
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GEOSTROPHIC WIND
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A steady horizontal motion of air along straight, parallel isobars or contours in an unchanging pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.
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GLACIER WINDS
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Air flow that descends from glaciers, occasionally at a high rate of speed. Caused by the temperature difference between the air in contact with the glacier and the air at the same altitude, it reaches maximum intensity in the early afternoon.
Related term: katabatic wind |
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GLAZE
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A smooth clear icy coating of supercooled water droplets that spread out and freeze onto objects on contact. A storm that produces the accretion of glaze is called an ice storm.
Related term: clear ice |
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GRADIENT WIND
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A steady horizontal air motion along curved parallel isobars or contours in an unchanging pressure or contour field, assuming there is no friction and no divergence or convergence.
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GRAUPEL
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A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together.
Related term: snow pellets |
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GRAVITATION
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The mutual attraction between two masses of matter. The rotation of the earth and the atmosphere modifies this attraction to produce the field of gravity.
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GRAVITY
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The force of attraction of the earth on an object. The direction is downward relative to the earth, and it decreases with elevation or altitude away from the earth's surface.
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GREEN FLASH
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A brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon.
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GREENHOUSE EFFECT
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The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space.
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GREENWICH MEAN TIME (GMT)
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The name of the twenty-four hour time scale which is used throughout the scientific and military communities. Standard Time begins at Greenwich, England, home of the Royal Observatory which first utilized this method of world time. This is also the Prime Meridian of Longitude. The globe is divided into twenty-four (24) time zones of 15 degrees of arc, or one hour in time apart. To the east of this meridian, time zones are number from 1 to 12 and prefixed with a minus (-), indicting the number of hours to be subtracted to obtain Greenwich Time (GMT). To the west, the time zones are also numbered 1 through 12, but are prefixed with a plus (+), indicating the number of hours to be added to obtain GMT.
Related terms: Universal Time Coordinate (UTC) and Zulu (Z) |
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GROUND CLUTTER
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A pattern of radar echoes reflecting off fixed ground targets such as buildings or hills near the radar. This may hide or confuse the proper return echo signifying actual precipitation.
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GROUND FOG
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Fog created when radiational cooling at the earth's surface lowers the temperature of the air near the ground to or below its initial dew point. Primarily takes place at night or early morning.
Related term: radiation fog |
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GROWING SEASON
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Considered the period of the year during which the temperature of cultivated vegetation remains sufficiently high enough to allow plant growth. Usually considered the time period between the last killing frost in the spring and the first killing frost of the autumn. The frost-free growing season is between the first and last occurrence of 32°F temperatures in spring and autumn.
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GULF STREAM
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The warm, well-defined, swift, relatively narrow ocean current which exists off the east coast of the United States, beginning near Cape Hatteras. The term also applies to the oceanic system of currents that dominate the western and northern Atlantic Ocean: the Florida current, which flows through the Florida Straits between the Florida Keys and Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras and flows northeasterly off the continental slope into the North Atlantic; and the North Atlantic current, which begins around the Grand Banks off Newfoundland and continues east-northeastwards towards the British Isles.
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GULLY WASHER
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A heavy rain shower that occurs suddenly, possibly creating a flash flood.
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GUST
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A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.
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GUST FRONT
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The leading edge of the cool, gusty surface winds produced by thunderstorm downdrafts. Sometimes confused with an outflow boundary.
Related term: first gust |
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GUSTNADO
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A weak, and usually short-lived, tornado that forms along the gust front of a thunderstorm, appearing as a temporary dust whirl or debris cloud.
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KATABATIC WIND
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A wind that is created by air flowing downhill. When this air is warm, it may be called a foehn wind, and regionally it may be known as a Chinook or Santa Ana. When this air is cold or cool, it is called a drainage wind, and regionally it may be known as a mountain breeze or glacier wind. The opposite of an anabatic wind.
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KATAFRONT
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A front where the warm air descends the frontal surface, except in the low layers of the atmosphere.
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KELVIN TEMPERATURE SCALE
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A temperature scale with the freezing point of +273°K (Kelvin) and the boiling point of +373° K. It is used primarily for scientific purposes. Also known as the Absolute Temperature Scale. Proposed in 1848 by William T. Kelvin, 1st Baron of Largs (1824-1907), Irish-born Scottish physicist and mathematician.
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K INDEX
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The measure of thunderstorm potential based on the vertical temperature lapse rate, the moisture content of the lower atmosphere and the vertical extent of the moist layer.
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KNOT
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A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. A knot is equivalent to 1.151 statute miles per hour or 1.852 kilometers per hour.
Related term: Dave's Dictionary |
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OBSCURATION
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Any phenomena in the atmosphere, excluding precipitation, that reduces horizontal visibility. According to the National Weather Service, some of the obstructions to visibility include blowing and widespread dust, fog (including freezing fog and patchy fog), haze, mist, sand and blowing sand, smoke, blowing spray, and volcanic ash. It is reported as "X" in an observation and on the METAR.
Related term: partial obscuration |
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OBSERVATION
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In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. An observer is one who records the evaluations of the meteorological elements.
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OCCLUDED FRONT
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Also known as an occlusion, it is a complex front formed when a cold front overtakes a warm front. It develops when three thermally different air masses conflict. The type of frontal boundary they create depends on the manner in which they meet.
Related terms: cold front and warm front |
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OCEAN
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The intercommunicating body of salt water occupying the depressions of the earth's surface, or one of its major primary subdivisions, bounded by the continents, or the equator, and other imaginary lines. A sea is subdivision of an ocean.
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OCEANOGRAPHY
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The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.
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OMEGA BLOCK
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A warm high aloft which has become displaced and is on the polarward side of the jet stream. It frequently occurs in the late winter and early spring in the Northern Hemisphere. The name comes from its resemblance to the Greek letter, Omega, when analyzed on upper air charts.
Related term: blocking high |
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OPAQUE
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A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.
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OROGRAPHIC LIFTING
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Where the flow of air is forced up and over barriers such as highlands or mountains. Moist air being forced aloft begins to cool, consequently condensation forms, and rain or snow begins to fall. By the time the air reaches the leeward side of the barrier, it sinks and warms, resulting in decreasing relative humidity, cessation of precipitation, and the dissipation of clouds. May be called an orographic uplift.
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OUTFLOW
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Also referred to as an outflow boundary, it is the outward flow of air from a system, such as a thunderstorm. It is the result of cold downdrafts and its passage includes a wind shift and temperature drop.
Related terms: bubble high and meso high |
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OVERCAST
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The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer.
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OVERRUNNING
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This occurs when a relatively warm air mass is forced above a cooler air mass of greater density. Weather generally associated with this event includes cloudiness, cool temperatures, and steady precipitation.
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OXYGEN (O2)
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A colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, comprising 20.946%.
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OZONE (O3)
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A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen molecule made up of three oxygen atoms instead of two.
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OZONE LAYER
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An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface
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TELECONNECTIONS
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Information used by forecasters to determine what the weather might be elsewhere when compared with past weather conditions at the same degree of longitude.
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TEMPERATE CLIMATE
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Climates with distinct winter and summer seasons, typical of regions found between the Tropics of Cancer and Capricorn and the Arctic and Antarctic Circles. Considered the climate of the middle latitudes.
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TEMPERATURE
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The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is also the degree of hotness or coldness. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.
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TERMINAL DOPPLER WEATHER RADAR (TDWR)
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Doppler radar installed at major airports throughout the United States to detect microbursts.
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TERRESTRIAL RADIATION
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Long wave radiation that is emitted by the earth back into the atmosphere. Most of it is absorbed by the water vapor in the atmosphere, while less than ten percent is radiated directly into space.
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TEXAS NORTHER
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Local name in the south-central Great Plains for strong winter winds blowing north or northwest following a sharp cold front with dropping temperatures. Marked by a dark, blue-black sky.Related term: Blue Norther
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THAW
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A warm spell of weather when ice and snow melt. To free something from the binding action of ice by warming it to a temperature above the melting point of ice.
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THEODOLITE
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An optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevation and azimuth angles.
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THERMAL LOW
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Also known as heat low, it is an area of low pressure due to the high temperatures caused by intensive heating at the surface. It tends to remain stationary over its source area, with weak cyclonic circulation. There are no fronts associated with it. An example is the low that develops over southwestern United States and northwestern Mexico during the summer months.
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THERMOCLINE
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A vertical negative temperature gradient in some layer of a body of water which is appreciably greater than the gradients above and below it. In the ocean, this may be seasonal, due to the heating of the surface water in the summer, or permanent.
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THERMODYNAMICS
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Study of the processes that involve the transformation of heat into mechanical work, of mechanical work into heat, or the flow of heat from a hotter body to a colder body.
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THERMOGRAPH
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Essentially, a self-recording thermometer. A thermometer that continuously records the temperature on a chart.
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THERMOHALINE
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In oceanography, it pertains to when both temperature and salinity act together. An example is thermohaline circulation which is vertical circulation induced by surface cooling, which causes convective overturning and consequent mixing.
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THERMOMETER
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An instrument used for measuring temperature. The different scales used in meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.
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THERMOSPHERE
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A thermal classification, it is the layer of the atmosphere located between the mesosphere and outer space. It is a region of steadily increasing temperature with altitude, and includes all of the exosphere and most, if not all, of the ionosphere.
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THICKNESS
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The thickness of a layer in the atmosphere is proportional to the mean temperature of that whole layer. The layer most often used in meteorology is between 1000 and 500 millibars. There can be different temperature profiles in the lowest layer of the atmosphere with the same 1000-500 millibar thickness value, depending on what is happening above that lowest layer. For example, if the lower levels are warming but higher levels are cooling, the overall mean temperature, the thickness, could remain the same. Likewise, on a sunny day, the amount of incoming solar radiation, affects the temperature right at the earth's surface, without necessarily having much effect on the thickness of the whole layer.
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THUNDER
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The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. Temperatures can rise to over 10,000 °C in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.
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THUNDER SNOW
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A wintertime thunderstorm from which falls snow instead of rain. Violent updrafts and at or below freezing temperatures throughout the atmosphere, from surface to high aloft, discourage the melting of snow and ice into rain. Intense snowfall rates often occur during these situations.
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THUNDERSTORM
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Produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes.
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TIDE
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The periodic rising and falling of the earth's oceans and atmosphere. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. This propagates a wave through the atmosphere and along the surface of the earth's waters.
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TILT
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The inclination to the vertical of a significant feature of the pressure pattern or of the field of moisture or temperature. For example, midlatitide troughs tend to display a westward tilt with altitude through the troposphere.
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TIROS
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A series of Television InfraRed Observation Satellites that demonstrated the feasibility and capability of observing the cloud cover and weather patterns of earth from space. An experimental program, it was the first spaceborne system that allowed meteorologists to acquire information that was immediately put to use in an operational setting. The first U.S. weather satellite, TIROS I, was launched on April 1, 1960, and TIROS X, the last of the series, was launched on July 2, 1965.
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TORNADO
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A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.
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TORNADO ALLEY
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A geographic corridor in the United States which stretches north from Texas to Nebraska and Iowa. In terms of sheer numbers, this section of the United States receives more tornadoes than any other.
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TOWERING CUMULUS
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Another name for cumulus congestus, it is a rapidly growing cumulus or an individual dome-shaped clouds whose height exceeds its width. Its distinctive cauliflower top often mean showers below, but lacking the characteristic anvil of a cumulonimbus, it is not a thunderstorm.
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TRACE
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Generally, an unmeasurable or insignificant quantity. A precipitation amount of less than 0.005 inch
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TRADE WINDS
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Two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. Primarily lower level winds, they are characterized by their great consistency of direction. In the Northern Hemisphere, the trades blow from the northeast, and in the Southern Hemisphere, the trades blow from the southeast.
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TRAJECTORY
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The curve that a body, such as a celestial object, describes in space. This applies to air parcel movement also.
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TRANSLUCENT
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Not transparent, but clear enough to allow light to pass through.
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TRANSMISSOMETER
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An electronic instrument system which provides a continuous record of the atmospheric transmission between two fixed points. By showing the transmissivity of light through the atmosphere, the horizontal visibility may be determined.
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TRANSPARENT
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A condition where a material is clear enough not to block the passage of radiant energy, especially light.
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TRANSPIRATION
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The process by which water in plants is transferred as water vapor to the atmosphere. Related terms: evapotranspiration
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TRIPLE POINT
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The point at which any three atmospheric boundaries meet. It is most often used to refer to the point of occlusion of an extratropical cyclone where the cold, warm, and occluded fronts meet. Cyclogenesis may occur at a triple point. It is also the condition of temperature and pressure under which the gaseous, liquid, and solid forms of a substance can exist in equilibrium.
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TROPICS/TROPICAL
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The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitiation during part of the year.
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TROPICAL AIR MASS
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An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry.
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TROPICAL CYCLONE
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A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.
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TROPICAL DEPRESSION
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A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize.
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TROPICAL DISTURBANCE
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An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.
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TROPICAL PREDICTION CENTER (TPC)
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A division of the National Centers for Environmental Prediction, the Center issues watches, warnings, forecasts, and analyses of hazardous weather conditions in the tropics for both domestic and international communities. The National Hurricane Center is a branch.
For further information, contact the TPC, located in Miami, Florida. |
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TROPICAL STORM
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A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the system is given a name to identify and track it.
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TROPICAL WAVE
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Another name for an easterly wave, it is an area of relatively low pressure moving westward through the trade wind easterlies. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.
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TROPIC OF CANCER
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The most northern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees North latitude.
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TROPIC OF CAPRICORN
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The most southern point on the earth where the sun is directly overhead, located at approximately 23.5 degrees South latitude.
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TROPOPAUSE
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The boundary zone or transition layer between the troposphere and the stratosphere. This is characterized by little or no increase or decrease in temperature or change in lapse rate with increasing altitude.
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TROPOSPHERE
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The lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. Characterized by clouds and weather, temperature generally decreases with increasing altitude
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TROUGH
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An elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic circulation. The opposite of a ridge.
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TSUNAMI
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An ocean wave with a long period that is formed by an underwater earthquake or landslide, or volcanic eruption. It may travel unnoticed across the ocean for thousands of miles from its point of origin and builds up to great heights over shallower water. Also known as a seismic sea wave, and incorrectly, as a tidal wave.
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TULE FOG
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Ground fog in the central valley of California and the leading cause of weather-related casualties in that state. It forms at night and in the early morning when the ground cools, lowering the air temperature near the ground to or below its initial dew point. Related term: radiation fog
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TURBULENCE
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The irregular and instantaneous motions of air which is made up of a number of small of eddies that travel in the general air current. Atmospheric turbulence is caused by random fluctuations in the wind flow. It can be caused by thermal or convective currents, differences in terrain and wind speed, along a frontal zone, or variation in temperature and pressure.
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TWILIGHT
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Often called dusk, it is the evening period of waning light from the time of sunset to dark. The time of increasing light in the morning is called dawn. Twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. Civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. Nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. Astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. During dawn, the reverse order occurs until full daylight.
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TWISTER
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A slang term used in the United States for a tornado.
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TYPHOON
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The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western North Pacific Ocean. This same tropical cyclone is known as a hurricane in the eastern North Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean.
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VALLEY BREEZE
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An anabatic wind, it is formed during the day by the heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up the mountainsides. It blows in the opposite direction of a mountain breeze.
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VAPOR PRESSURE
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The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.
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VAPOR TRAIL
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A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. Also called a contrail, for condensation trail.
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VARIABLE CEILING
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Occurs when the height of a ceiling layer increases and decreases rapidly, The ascribed height is the average of all the varying values.
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VEERING
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A clockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is counterclockwise. This can either happen horizontally or vertically (with height). For example, the wind shifts from the north to the northeast to the east. It is the opposite of backing.
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VERNAL EQUINOX
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Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is the astronomical opposite of the autumnal equinox.
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VERTICAL TEMPERATURE PROFILE
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A series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.
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VERTICAL VISIBILITY
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The distance an observer can see vertically into an undefined ceiling, or the height corresponding to the top of a ceiling light projector beam, or the height at which a ceiling balloon disappears during the presence of an indefinite ceiling.
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VERTICAL WIND PROFILE
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A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.
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VIRGA
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Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus, or high based cumuonimbus.
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VISIBILITY
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A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visibility conditions at the station. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. Tower visibility is the prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.
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VISIBLE LIGHT
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The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 mile per second. It has a wave length longer than ultraviolet light and shorter than x-rays.
Related term: light waves |
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VISUAL FLIGHT RULES (VFR)
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Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles.
Related terms: MVFR and IFR |
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VORTEX
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Any circular or rotary flow in the atmosphere that possesses vorticity.
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VORTICITY
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The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.
Related term: relative vorticity |
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VORTICITY MAXIMUM
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A center of vorticity, or the maximum of the vorticity field of a fluid.
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VALLEY BREEZE
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An anabatic wind, it is formed during the day by the heating of the valley floor. As the ground becomes warmer than the surrounding atmosphere, the lower levels of air heat and rise, flowing up the mountainsides. It blows in the opposite direction of a mountain breeze.
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VAPOR PRESSURE
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The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.
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VAPOR TRAIL
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A cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. A vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. Also called a contrail, for condensation trail.
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VARIABLE CEILING
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Occurs when the height of a ceiling layer increases and decreases rapidly, The ascribed height is the average of all the varying values.
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VEERING
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A clockwise shift in the wind direction in the Northern Hemisphere at a certain location. In the Southern Hemisphere, it is counterclockwise. This can either happen horizontally or vertically (with height). For example, the wind shifts from the north to the northeast to the east. It is the opposite of backing.
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VERNAL EQUINOX
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Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is the astronomical opposite of the autumnal equinox.
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VERTICAL TEMPERATURE PROFILE
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A series of temperature measurements taken at various levels in the atmosphere that show the thermal structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.
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VERTICAL VISIBILITY
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The distance an observer can see vertically into an undefined ceiling, or the height corresponding to the top of a ceiling light projector beam, or the height at which a ceiling balloon disappears during the presence of an indefinite ceiling.
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VERTICALWIND PROFILE
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A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.
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VIRGA
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Streaks or wisps of precipitation, such as water or ice particles, that fall from clouds but evaporate before reaching the ground. From a distance, the event sometimes may be mistaken for a funnel cloud or tornado. Typically, it may fall from altocumulus, altostratus, or high based cumuonimbus.
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VISIBILITY
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A measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. The National Weather Service has various terms for visibility. Surface visibility is the prevailing visibility determined from the usual point of observation. Prevailing visibility is considered representative of visibility conditions at the station. Sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. Tower visibility is the prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.
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VISIBLE LIGHT
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The portion of the electromagnetic spectrum that can be detected by the human eye. It travels at the same speed as all other radiation, that is at 186,000 mile per second. It has a wave length longer than ultraviolet light and shorter than x-rays.
Related term: light waves |
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VISUAL FLIGHT RULES (VFR)
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Refers to the general weather conditions pilots can expect at the surface. VFR criteria means a ceiling greater than 3,000 feet and greater than 5 miles.
Related terms: MVFR and IFR |
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VORTEX
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Any circular or rotary flow in the atmosphere that possesses vorticity.
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VORTICITY
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The measurement of the rotation of a small air parcel. It has vorticity when the parcel spins as it moves along its path. Although the axis of the rotation can extend in any direction, meteorologists are primarily concerned with the rotational motion about an axis that is perpendicular to the earth's surface. If it does not spin, it is said to have zero vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel has a counterclockwise, or cyclonic, rotation. It is negative when the parcel has clockwise, or anticyclonic, rotation.
Related term: relative vorticity |
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VORTICITY MAXIMUM
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A center of vorticity, or the maximum of the vorticity field of a fluid.
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YEAR
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The interval required for the earth to complete one revolution around the sun. A sidereal year, which is the time it take for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, the Gregorian calendar of 365 days is used, with 366 days every four years, a leap year. The tropical year, also called the mean solar year, is dependent on the seasons. It is the interval between two consecutive returns of the sun to the vernal equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century.
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YELLOW SNOW
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Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.
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