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MET RevisionChapter 1. The Atmosphere
1. The atmosphere comprises 78% nitrogen, 21% oxygen and 1% of other gases. (Mainly argon
0.93% and CO, 0.05% and other trace gases including water vapour).
2. In the troposphere, temperature decreases with height at approximately 2 °C/1000ft or 0.65 ° C/100
m up to the tropopause - which has an an average height of 11km. The troposphere contains almost
all the water vapour and weather pertinent to aviation.
3. Over the poles the cold air causes the tropopause to be lower at between FL 250 FL 320 (8-10 km).
Over the equator the warm air causes the tropopause to be higher between FL 500 - FL 600 (16-18
km)
4. Especially over land, the large changes in temperature between winter and summer brings the
tropopause down to FL 220 in the Siberian winter (high pressure) and raises it up to FL 600 over SE
Asia during the summer monsoon Asian low
5. The stratosphere is the laver above the troposphere. Its temperature is constant with height up to
20 km (FL 650). Above this, the temperature increases with height due to the production of ozone
6. Ozone (O3) is a powerful oxidant that can damage the respiratory system so it must be removed
from cabin air at the high altitudes typical with the cruise altitudes of commercial and corporate jet
aircraft
7. The ICAO International Standard Atmosphere (ISA) provides a common mean reference for
temperature, pressure and density. It is a standard against which we model, calibrate and compare
against the actual atmosphere at any point and time
8. In ISA at mean sea level (MSL) the temperature is 15°C and the air cools at approx. 2° C/1000 ft
9. At 2000 ft the ISA temperature is 11°C. If the actual temperature is, for example, 6 °C then the ISA
deviation is ISA-5
10. ISA Conditions:
• MSL pressure is 1013.25 hPa.
• MS temperature is 15°C.
• MSL temperature lapse rate is 0.65°C per 100 m or 1.98°C per 1000 ft up to 11 km where the
temperature decreases to -56.5° C.
• The Tropopause is at 11 km and it's temperature is -56.5°C.
• From 11 km to 20 km the temperature lapse rate is zero (isothermal) and stays at -56.5°C.
, 11. The tropopause isotherm and or stratospheric inversion represent significant stability, and the
stability only increases beyond there for tens of kilometres vertically. This creates a barrier to the
further ascent of air and therefore the further vertical production of cloud.
Chapter 2. Heating
1. Terrestrial radiation from the Earth to the air is the primary heat source for the atmosphere. This is
one reason why the temperature decreases with height
2. Due to differential heating and cooling of the Earth's surface, there are differential densities,
horizontally, which cause the air to move. This is the primary means of heat transfer in the
atmosphere
3. Radiation is the emission or transmission of energy in the form of waves. The main sources of
radiation are the Sun and the Earth; solar and terrestrial radiation, respectively:
Solar radiation is intense and short wave.
Terrestrial radiation is low intensity, long wave radiation.
4. Reflection, scattering and absorption are atmospheric processes that filter and modify the solar
radiation passing through our atmosphere.
5. Greenhouse gases (water vapour, CO, and methane), absorb long wave terrestrial radiation and
then re-emit it. This adds heat and energy to the atmosphere.
6. Convection is the transfer of heat energy due to the movement of matter, in this case, the air.
Convection is a primary means of transferring energy vertically through the atmosphere
Advection is a transfer of energy by the horizontal movement of the air.
Advection occurs whenever the wind blows.
7. Latent heat is "hidden' heat energy which is released or absorbed to change the state of a
substance without changing its temperature.
8. Isothermal layers and inversions (where temperature remains constant with altitude or increases
with altitude) create absolutely stable conditions which inhibit the vertical motion of air.
9. Ground inversions are formed either at night as the surface layers cool, or at other times by
warmer air moving over a cooler surface.
10. Subsidence inversions are common in high pressure areas, where the descending dry air warms
faster than the static environmental air.
, 11. Frontal inversions form when warm less dense air flows up and over cooler, denser air.
12. An isotherm is a line of equal temperature or a region of constant temperature with height.
13. Convection occurs whenever the air is heated or cooled. Warmer air is less dense and, thus, rises.
Cooler air is denser and, thus, sinks. Rising and sinking air is the cause of convection currents.
Chapter 3. Pressure
1. Isobars are lines of equal (constant) pressure on a pressure map. They are drawn as a long,
continuous black line and are usually given a value in even numbers.
2. A high-pressure, high (anticyclone) is a region where the atmospheric pressure at the surface is
greater than its surrounding environment
3. A ridge is an extension of a high, characterised by a protrusion of isobars away from the centre of
the high.
4. A low-pressure is an area in which the atmospheric pressure is less than its surroundings. A trough
is its extension.
5. In standard conditions, the pressure lapse rate is approximately 30 ft per Pa at mean sea level, 50 ft
per hPa at 20 000 ft and 73 ft per hPa at 30 000 ft.
6. In ISA conditions, pressure is approximately 50 % of MSL at 18 000 ft
Chapter 4. Density
1. Density is approximately 50 % of MSL at 22 000 ft and 25 % of MSL at 40 000 ft.
2. Density is inversely proportional to temperature. As temperature increases⬆️, density
decreases⬇️.
3. Density decreases with height at a decreasing rate. Density decreases rapidly at lower levels and
slowly at higher levels.
Chapter 5. Pressure
1. The 2 main types of pressure system are: the Warm and Cold anticyclones
Warm anticyclones form from high level convergence forcing subsidence from above
(perhaps from the outflow of adjacent low pressures) which then reaches the surface and
diverges.
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