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51 Cards in this Set
- Front
- Back
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Polar‐Front Theory
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theory that explains the life cycle of mid-latitude cyclones and their associated fronts.
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Stages of cyclone life cycle: 1-3
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1. Stationary: separates cold polar air from warm sub-tropical air
2. Frontal Wave: the initial kink that forms from the polar front (shaded area = precipitation) 3. Open Wave: a fully developed cyclone (tight isobars = stronger cyclonic flow) |
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Stages of a cyclone life cycle: 4-6 |
4. Mature: pressure drops, wind increases, cold front inches closer to the warm front
5. Advanced Occlusion: cold front is caught up to the warm front leaving cold air at the surface 6. Dissipation: cyclone loses energy and gradually dies from the lack of temperature gradient at the surface |
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Speed of fronts
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cold fronts move faster than warm fronts
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Troughs and ridges
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Trough: elongated area of low pressure that accounts for the wind shift in a cold front
Ridge: elongated region of relatively high atmospheric pressure, the opposite of a trough |
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Convergence and divergence
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Convergence: moving together
Divergence: moving apart |
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Convergence and divergence at upper-levels and at the surface
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Surface high = surface divergence, upper level convergence
Surface low = surface convergence, upper level divergence Convergence: adds mass to a column of air; caused by winds towards each other OR a faster wind approaching a slower wind. Upper-level convergence happens on the western side of a trough Divergence: removes mass from a column of air; caused by winds blowing in opposite directions OR a slower wind behind a faster wind. Upper-level divergence happens on the eastern side of a trough |
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Types of mid‐latitude cyclones (i.e. nor’easter)
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Lee-side low: westerly winds blow over a mountain range, air expands vertically on the downwind side with intensifies pre-existing low pressure (causes the cyclone to form on the eastern slope)
Nor’easters: an east coast mid-latitude cyclone that develops or intensifies off the east coast of North America in the fall, winter, or spring. Strong winds, heavy rain/snow/sleet. Caused by warm ocean temperatures and cold land temperatures. Gain additional energy from moisture that forms over the ocean |
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Chapter 9: What is a weather forecast? |
A scientific estimate of the weather conditions at some future time
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Weather organizations
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World Meteorological Organization (WMO), National Weather Service (NWS),
National Centers for Environmental Prediction (NCEP). |
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Difference between a watch, a warning, and an advisory
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Watch: indicates that atmospheric conditions favor hazardous weather occurring over a particular region during a specified time period, but the actual location and time of the occurrence is uncertain.·
Warning: indicates that hazardous weather is either imminent or actually occurring. Advisory: used to inform the public of less hazardous conditions caused by wind, dust, fog, snow, sleet, or freezing rain. |
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How do we observe weather?
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On a global scale there are over 10,000 observation stations on land and about 7,000 observation ships. In the United States there are 119 Weather Forecast Offices, the Federal Aviation Administration (FAA) and NWS operate about 900 automated surface observing systems.
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Types of observations
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there are upper air observations that are recorded by Radiosondes
Data buoy: used to record atmospheric conditions |
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Satellites – polar and geostationary
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Polar: circle the Earth in a north-south direction· Geostationary: placed in orbit over the equator; remain at a fixed point above the Earth because they travel at the same rate that the Earth rotates.
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Satellite images
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Infrared: obtained by detecting the radiation emitted by objects (amount of radiation and type of wavelength is dependent on the temperature)
Water Vapor: measures how much water vapor is in the air by detecting the wavelength emitted by water vapor |
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What do we use to report the weather?
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Synoptic weather maps: give a picture of the weather at a given point in time
Meteograms: a chart that shows how one or more weather variables has changed at a station over a given period of time Soundings: two dimensional vertical profile of temperature, dewpoint, and winds |
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Numerical weather prediction, MOS, ensemble forecasting
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Numerical weather prediction: technique used to forecast weather using numerical models designed to represent atmospheric processes
MOS: a process for forecasting that corrects for model tendencies Ensemble forecasting: a technique used for forecasting that produces multiple forecasts using the same model with slightly different initial conditions |
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Types of forecasting methods (i.e. persistence, analog, etc.)
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Persistence: based on the tendency of weather to remain unchanged for several hours of days.
Climatological: uses the average weather statistics measured over many years Analog: based on the assumption that weather repeats itself; involves pattern recognition Trend: based on the speed and direction of features such as fronts, cyclones, clouds and precipitation (also known as nowcasting) Statistical: made routinely of weather elements based on the past performance of computer models; use the MOS Probability: provides the probability that something will happen; i.e. a “white Christmas” |
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Types of forecasts (i.e. nowcast, short-range, etc.)
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Nowcast: a forecast that is good for a few hours
Short-range: a forecast that is good from about 6 hours to a few days Medium-range: a forecast that is good from about 3-8 days Long-range: a forecast that extends beyond 8 days. Usually provided by the Climate Prediction Center. |
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What determines the movement of a weather system?
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Mid-latitude cyclonic storms and fronts tend to move in the same direction at approx. the same speed that they did during the previous six hours
Low-pressure areas tend to move in a direction that parallels the isobars in the warm air (the warm sector) ahead of the cold front. Lows tend to move towards the region of greatest surface pressure drop, whereas highs tend to move toward the region of greatest surface pressure rise Surface pressure systems tend to move in the same direction as the wind at the 500 mb level. The speed at which surface systems move is about half the speed of the winds at this level. |
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Chapter 10: Thunderstorms |
storms that generate lightning and thunder. Usually produces gusty winds, heavy rain and hail.
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Ordinary (Air‐mass) Thunderstorms
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form in regions with rising air parcels and very little vertical wind shear. Commonly found in mountainous regions and near the intertropical convergence zone (ITCZ).
Has 3 stages: Cumulus stage: moist rising air creates cumulus clouds, water vapor condenses into liquid or solid cloud particles which releases large quantities of latent heat, a process that keeps the rising air inside the cloud warmer than the air around it. In this stage there is no precipitation yet, the updrafts keeps the water up in the cloud. Mature Stage: Falling precipitation creates a downdraft, this downdraft causes entrainment bringing drier air into the cloud. The entrainment causes evaporation, which effectively cools the air enhancing the downdraft. At this stage there is both an updraft and a downdraft. An anvil will form if the cloud top reaches the top of the unstable region of air. Where the cold downdraft reaches the surface the air spreads out horizontally in all directions. Then a gust front forms, which is the surface boundary that separates the advancing cooler air from the surrounding warmer air. Dissipating Stage: Begins about 15-30 minutes after the mature stage starts. The downdraft overpowers the updraft, cutting off the source of the warm rising air that was fueling the thunderstorm. |
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Severe Thunderstorms
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contains at least one of the following criteria: winds that exceed 58 mph, hailstones larger than .75 inches in diameter, and/or a tornado forms. The processes are as follows:
-Some key factors are the presence of wind shear, the updraft does not remain vertical, and the precipitation falls in the downdraft, not the updraft. -The updraft continues to strengthen and commonly, overshoots into the stratosphere.· Downdrafts at the surface act as a wedge to create a gust front and lift the warm, moist air into the updraft. |
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Supercell Thunderstorms
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made of a single, very powerful cell with vertical heights that extend to 20 km. These storms persist for many hours and have diameters that reach 12-30 miles across. The rotation occurs in the updraft due to strong vertical wind shear.
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Squall lines
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a relatively narrow band of thunderstorms that develop in the warm sector of a mid-latitude cyclone. Usually advance a cold front by 100-300 km.
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Roll clouds
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appear to spin about a horizontal axis, elongated cloud that forms behind the gust front.
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Downbursts
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a localized downdraft that hits the ground and spreads horizontally in a radial burst of wind. (think about water pouring from a s tap and striking the sink below)
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Flash floods
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floods that rise rapidly with little or no advanced warning. Usually happen when thunderstorms stall or move very slowly, causing heavy rainfall over a relatively small area.
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Random, turbulent eddies that lift small bubbles of air
Unequal heating at the surface The effect of terrain or the lifting of air along shallow boundaries of converging surface winds Diverging upper-level winds, coupled with converging surface winds and rising air Large scale uplift along mountain barriers or gently rising terrain Warm air rising along a frontal zone |
What causes rising motion?
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Lightning and thunder
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-Lightning: a discharge of electricity, a giant spark, that occurs in mature thunderstorms. Can heat the air around it to 54,000°F
-Occurs when there is an imbalance of positive and negative charges in a cloud; negative at the bottom and positive at the top. -The positively charged ground attracts the negative charges at the base of the cloud. When the electrical potential gradient becomes large enough to overcome the air’s strong insulating power a flow of electrons called the stepped leader rushes to the ground. -As the stepped leader approaches the ground a region of positive charge moves up into the air through a conducting object. After these opposing charges meet there is a strong electrical current called the return stroke that carries the positive charge into the cloud and produces the bright spark. This process repeats itself several times through subsequent leaders called dart leaders; can be a lot of dart leaders in a single spark. -Thunder: The extreme heat from lightning causes the air to expand explosively, initiating a shock wave that becomes a booming sounds traveling outward in all directions from the flash. |
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What is a tornado?
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A rapidly rotating column of air extending down from a cumulus cloud, a small area of intense low pressure, to the ground.
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What are the different parts of a tornado? (i.e. wall cloud, funnel cloud, etc.)
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-Wall Cloud: area of rotating clouds that extends beneath a supercell thunderstorm & from which a funnel cloud may appear.
-Funnel Cloud: tornado whose circulation has not yet reached the ground, extends from the base of a thunderstorm |
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What the life cycle of a tornado?
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-Dust-whirl stage: dust swirling upward from the surface marks the tornado’s circulation on the ground and a short funnel often extends downward from the thunderstorm’s base.
-Mature stage: during this stage, damage is usually most severe as the funnel reaches its greatest width and is almost vertical. -Decay stage: the tornado stretches into the shape of a rope and dissipates. |
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What are the ideal conditions for the formation of a tornado?
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Warm, humid surface air that is overlain with cooler, drier air aloft. These conditionally unstable conditions combined with a strong vertical wind shear
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Watches vs. warnings
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-Watch: alert the public to the possibility of tornadoes over a specified area for a particular time interval. Predict the organized sever weather event where the tornado will affect at least 10,000 square miles and/or persists for at least 3 hours.
-Warning: alert the public when a tornado has actually been sighted in an area or is indicated by weather radar. Pertain to a specific county of counties and last for about 30-60 minutes. |
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How are tornadoes classified?
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They are classified by the Enhanced Fujita Scale which is a scale that uses the damage caused by the tornado to estimate the tornado wind speeds.
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How do tornadoes form?
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-Many tend to form with intense thunderstorms, most often with supercell thunderstorms in an environment with strong vertical wind shear.
-Conditionally unstable atmosphere is essential for their development. |
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When do tornadoes form?
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In the early summer, peaking in the month of May.
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Chapter 11: Hurricanes |
a tropical cyclonic storm having minimum winds of 119 km/hr, also known as a typhoon (western Pacific) or a cyclone (Indian Ocean).
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Anatomy of a hurricane
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-Eye: a roughly circular area of relatively light winds and fair weather located at the center of the hurricane
-Eye wall: the doughnut-shaped area of intensive cumulonimbus development and very strong winds that surround the eye. -Spiral rainbands: clouds that align themselves into spiraling bands that swirl in toward the storm’s center -A hurricane is composed of an organized mass of thunderstorms. |
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Where do you have rising motion? Sinking motion? And what kind of weather would you expect with each?
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There is sinking motion in the eye and relatively calm weather, whereas there is rising motion in the eyewall and the surrounding spiral rainbands which is associated with rougher weather.
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What is needed for a hurricane to form?
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A cluster of thunderstorms must become organized around a central area of low pressure. Need warm water, humid air, converging winds. A trigger must start the air converging. |
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What will cause a hurricane to strengthen? Dissipate? |
Hurricanes survive when they remain over warm water and weaken rapidly when they travel over colder water, losing their energy source and forcing them to dissipate. Can also weaken if the layer of warm water beneath the storm is shallow. |
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Why don’t hurricanes form over the equator? |
Hurricanes need Coriolis acceleration to cause their rotation and the Coriolis Effect is 0 at the equator |
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What are the different stages for a hurricane? |
-Tropical disturbance: disorganized array of clouds and thunderstorms, have weak pressure gradients and no rotation. -Tropical depression: as pressure falls, the thunderstorms grow bigger and start to organize. As air flows toward the low pressure zone, it picks up more energy from the warm sea surface and also starts to rotate due to the Coriolis force. Wind speeds are between 23 and 39 mph. -Tropical storm: the system takes on a circular stage as it becomes more organized, with a clear center. Wind speeds are between 40 and 74 mpg and the storm receives a name. -Hurricane: the storm turns into a highly organized hurricane as wind speeds exceed 74 mph. The eye appears and is surrounded by the spiral rainbands. |
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What wind speeds are needed to classify a storm as a hurricane? |
Wind speeds that exceed 74 mph. |
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How are hurricanes classified? |
Category 1 is 74 - 95 mph and Category 5 is 157mph + Saffir-simpson scale |
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How are hurricanes named? |
During the tropical storm phase -Named alphabetically but omits Q, U, X, Y, and Z -Names from the Greek alphabet are used if the number of hurricanes extend the list -List recycles names every six years -Except if the hurricanes become famous (i.e. Sandy, Katrina) |
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Hurricane forecast (watch vs. warning) |
Once a tropical cyclone forms, the NHC continues to monitor its developmentIncludes track, intensity, size, storm, surges, rainfall, and associated tornadoes -Shaded areas are where the hurricane could be. Possibility for further storms -Lighter (4-5 days) darker (6 days +) -WatchAnnounced when a hurricane poses a possible threat 36 hours -Warning: Announced when a hurricane is sustained and exposed to the coast within 24 hours |
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Hurricane movement |
-Tend to form over the warm, tropical North Pacific and North Atlantic -Steered by the easterly trade winds -Move westward and poleward - unless in the Gulf and turn towards the equator -Arrows indicated the possible track and hurricanes can travel up to 58 mph -Hurricanes don’t tend to form on the west coast because hurricanes form in warm, low pressure systems -The movement of a hurricane differs given its location and the season |
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Hurricane destruction |
Storm surge: -When the eye of the hurricane makes landfall with a dome making a 40-50 mi wide sweep across the coast due to the piling up of ocean water by strong onshore winds -These are more intense on the right side of the eye in the Northern Hemisphere -Storm surges are less intensive when there are stronger onshore winds Wind damage: -Less severe than a storm surgeHowever, it affects a larger area -More than half the hurricanes that make landfall, produce at least one tornado (ex. Bonnie, Charley, Frances, Ivan, Jeanne) -Inland freshwater floodingHeavy rain and flooding can occur days after the storm makes landfall -Property damage, death |
