Meteorology Atmospheric Fundamentals

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The International Standard Atmosphere

Back 1

Because the temperature & pressure of the atmosphere is changing continuously , a standard atmosphere was produced by the International Civil Aviation Organisation (ICAO).

The standard atmosphere is based on long term world wide average values for the various atmospheric elements.

This standard atmosphere can be used as a basis to specify performance calibrate instruments and so on.

It is called the ISA (international Standard Atmosphere)

Front 2

ISA

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The main specifications of the ISA used in practice for within the troposphere are:

Mean sea level (MSL) pressure 1013 hPa

Pressure lapse rate - 30 ft per hPa ( approx value from sea level to 10,000 feet)

Mean sea level temperature. +15 deg C

Temperature lapse rate -2 deg Per 1000 ft ( constant up to approx 36,000 ft)

Lapse rate - the rate at which a value (pressure or temp ) decreases with increasing height

Front 3

Atmospheric pressure

Back 3

Even though air is very thin it still has weight, the weight of the air in the atmosphere exerts and average pressure on the Earth's surface of 1013 HPA. Hectopascal's the unit of pressure used in meteorology.

As we climb through the atmosphere weight of air above reduces. Therefore the pressure decreases as height is increased.

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Atmospheric pressure..cont..

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Initially the rate of pressure reduction is quite large. At about 18,000 feet, the atmospheric pressure will have reduced to about 500 HPa i.e., about half sea level value.

Atmospheric pressure changes with distance (horizontal )and time as well as with height see figure 2.1.1

In order to compare pressure at various places, the measured station level pressure is converted to an equivalent mean sea level pressure.

Therefore, regardless of height of the station, the measured station level pressure is converted to mean sea level pressure for that station so that it can be compared with the mean sea level pressure at other places.

Hint 4

Front 5

Atmospheric pressure ..continued

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At any place near sea level, the weight of the atmosphere is on average equal to the weight of a column of mercury 76 cm high. The type of instrument used in measuring atmospheric pressure is known as the mercury Barometer

The unit used in measuring atmospheric pressure is the Hectopascal HPA. The standard sea level atmospheric pressure is 1013 HPA which is equivalent to 76 cm of mercury

Another type of barometer used to measure atmospheric pressure is the aneroid barometer. This type uses a spring instead of a column of mercury to balance the weight of the atmosphere. They are not as accurate as mercury barometer ,the aneroid barometer is lighter more robust and transportable.the altimeter in an aircraft is a modified version of an android barometer.

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Atmospheric pressure continued

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The change in pressure from place to place and for a particular time can be shown on the map. This is done by joining lines through places that have the same value of mean sea level pressure. These lines joining places of equal surface pressure are called isobars.

The isobars indicate mean sea level pressure MSL on the Earth's surface in much the same way as contour lines on a map show the elevation of the Earth's surface or topography.

When 2 different places have different MSL pressures regardless of whether the places are at sea level or not, the pressure difference between them is known as the horizontal pressure gradient between the two places. The horizontal pressure gradient is defined as the rate of change in MSL pressure per unit of (horizontal )distance, e.g.

5 hPa per 100 miles

The MSL atmospheric pressure for places at sea level (and the equivalent mean sea level pressure for places above sea level )is given the q code abbreviation QNH. When QNH is set on the pressure settings subscale of an altimeter , the altimeter will indicate altitude, i.e., height above mean sea level in feet.

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QNH Elevation & Pressure Height (finding with altimeter)

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With the aircraft on the ground at an aerodrome , the following procedures should be used.

To Find local QNH..

Turn the subscale setting knob until the altimeter indicates the elevation of the aerodrome

The Reading on the pressure settings subscale is the local QNH

. To find the pressure height at the aerodrome, -set the pressure settings scale to 1013 HPA the reading on the main altimeter scale is the aerodrome pressure height.

.To find the elevation of an aerodrome (when the QNH is known,

set the pressure settings scale to the local or area QNH- the reading on the main Altimeter scale is the elevation of the aerodrome.

Note, one way to remember altimeter settings is that if you want a height (or elevation) set a pressure and if you want a pressure, set a height (or elevation. )

Height (elevation )always goes with the altimeter main scale and the pressure goes with the pressure subscale.

Front 8

Calculating or estimating the pressure height

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Aircraft performance depends on the density of the air, i.e., on both the pressure and the temperature.

When using performance charts to calculate take off and landing distance required and/or weights,

pressure is always specified from the common datum of ISA mean sea level i.e. 1013HPa

And The height is then known as pressure height

. When not in the aircraft where the altimeter is used, the pressure height at an aerodrome is calculated from the QNH and the elevation of the aerodrome.

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Calculating or estimating the pressure height

Back 9

Aircraft performance depends on the density of the air, i.e., on both the pressure and the temperature.

When using performance charts to calculate take off and landing distance required and/or weights,

pressure is always specified from the common datum of ISA mean sea level i.e. 1013HPa

And The height is then known as pressure height

. When not in the aircraft where the altimeter is used, the pressure height at an aerodrome is calculated from the QNH and the elevation of the aerodrome.

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Calculating or estimating the pressure height , continued..

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Pressure height pH is defined as the height above sea level in the international standard atmosphere ISA.

Another way of defining it is to say that pressure height is the reading on the Altimeter when the pressure setting sub-scale of the altimeter is set to 1013 HPA i.e., the ISA Sea level pressure.

The ISA MSL pressure datum is 1013 HPA and atmospheric pressure decreases by about 1HPA for each 30 feet of height increase in the lower levels of the atmosphere

When you are checking the takeoff landing distance required, it is unlikely that you will be in the aircraft. When you are away from the aircraft, you must know the QNH and the elevation (ELEV) of the aerodrome.

The QNH can be found either from the aerodrome forecast TAF, if one is issued for your aerodrome, or from an area forecast ARFOR for the area you are in.

The use of local QNH when available will give you a slightly more accurate result than when area QNH is used .either is acceptable.

The elevation if known can be found in Ersa or from a WAC chart of the area.

In practice, the pressure height is found by correcting the elevation of the aerodrome for the difference between the QNH and 1013 HPA so that;

PH = Elev + (dif between 1013 & QNH converted to feet)

= Elev + [(1013-QNH)x 30] when QNH is less than 1013; or

=Elev - [(QNH - 1013) x 30 when QNH is greater than 1013.

Since the pressure height only needs to be to the nearest 500 feet, i.e., plus/minus 250 ft, and easier method of estimating the pressure height is to use the following rules;

QNH above 1020 hpa-> use

PH = ELEV - 500 ft

QNH between 1005 & 1020 hPa ->. Use ELEV as PH

QNH below 1005 hPa -> use

PH = ELEV +500ft

Hint 10

Front 11

Find pressure height at an aerodrome (elevation 1500 feet) where the local QNH is 1005 hPa

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Estimated PH - since QNH is 1005, then PH =ELEV = 1500 ft( close enough)

Calculated PH = 1500 +[(1013-1005) x 30 ft]

= 1500 + [8x30] = 1500 + 240 = 1740 ft

That is, A pressure difference between 1013 and the QNH of 8HPA converts to 240 feet .

which is added to the elevation to attain the pressure height of 1740 feet.

Alternatively because the MSL pressure is lower than standard, the pressure height is higher than the elevation and is equivalent to the pressure at an aerodrome with an elevation of 1740 ft on a standard day, i.e., when the local QNH is 1013 HPA

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Find the PH at an aerodrome (elevation 1870 feet) where the local QNH is 1028 hPa

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Estimated PH - since QNH is 1028 then PH = ELEV - 500 =1370 ft ( close enough)

Calculated PH - 1870 - [(1028-1013) x 30 ft]

= 1870 -[15 x 30] = 1870 -450 =1420 ft

That is, a pressure difference between the QNH and 1013 of 15 HPA converts to 450 feet which is subtracted from the elevation to obtain the pressure height of 1420 feet. Alternatively because the MSL pressure is higher than standard the pressure height is lower than the elevation and is equivalent to the pressure at an aerodrome with an elevation of 1420 feet on a standard day, I.e., when the local QNH is 1013 HPA.

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The effects of increasing pressure height

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Generally , whenever pressure height increases , the density decreases and aircraft performance will decrease .

Density depends on both temperature and pressure but pressure has the greater influence - thus the statement that increasing pressure height decreases the density.