Skip to content
FIS Book
MET

MeteorologyLesson 2 of 4

Back to subjects
MET 002

The atmosphere

What the atmosphere is

A mixture of gases surrounding the earth. About 99% of its mass lies within the first 30 km above the surface.

78%Nitrogen
21%Oxygen
1%Other gases
Up to 4%Water vapour, in the lower atmosphere. The amount varies from place to place, which is why the three figures above are for dry air.

The layers

Five layers, each topped by a pause. What tells them apart is temperature: on the way up it falls, rises, falls and rises again.

ThermopauseMesopauseStratopause · 51 kmTropopause · 7–18 kmExosphereThermosphereMesosphereStratosphereTroposphereAuroraNoctilucent clouds76–85 kmOzoneWeather20 km−80 to −100 °CSurfaceTemperature
Not to scale. The red line is temperature, and it changes direction at the pauses.
From the surface up
TroposphereSurface to 7–18 km
  • Almost all the water vapour and weather is found in this layer.
  • Heated from below, which promotes vertical mixing of the air.
Tropopause
StratosphereUp to 51 km
  • Small vertical movement.
  • Temperature stable, then increasing with height from 20 km, due to the high amount of ozone.
Stratopause
Mesosphere
  • Temperature falls again, to between −80 and −100 °C. There is little ozone left to absorb the sun’s ultraviolet, and carbon dioxide radiates heat away.
  • No typical meteorological phenomena, but noctilucent clouds form at 76 to 85 km.
Mesopause
Thermosphere
  • The aurora borealis is up here.
  • Temperature rises again. The few molecules up here absorb the sun’s ultraviolet and X-ray radiation, and there are so few of them that it heats them to high temperatures.
Thermopause
Exosphere
  • The outermost layer.

The tropopause

The top of the troposphere, the layer that holds almost all the weather.

Pole: cold airabout 7 kmEquator:hot air,18 kmTropopause
High over the hot equator, low over the cold poles. Drawn far thicker than it is.
Not a hard boundaryIt works a little like a lid.
7 to 18 kmAbout 23 000 to 60 000 ft. It depends on the latitude and the season.
Found by temperatureIdentified by measuring the change of temperature with height.

Temperature, moisture and pressure

What each one is, what it is measured in, and how it changes with height.

Temperature
°CTemperatureDegrees Celsius, in aviation

A numerical measure of hot and cold. It usually decreases as altitude increases. Measured by a thermometer.

Moisture
%HumidityPer cent
°CDew pointDegrees Celsius

Water vapour: the driving force of the weather. Warm air can hold more water vapour than cold air.

Atmospheric pressure
hPaPressureHectopascals, in aviation

The force exerted against a surface by the weight of the earth’s atmosphere. Unlike temperature, it always decreases as altitude increases. Often expressed as mean sea level pressure. Measured by an electronic pressure sensor or an aneroid barometer.

Why it matters to aviation: it measures height
QFE indicates heightThe barometric pressure at the level of an aerodrome.
QNH indicates altitudeThe local pressure reduced to mean sea level according to the ISA.

Altimetry in full, with 1013.25 hPa, the third setting, and how to work with all three, is ATM 005.

The International Standard Atmosphere

To simplify the atmosphere, a standard one is assumed. The ISA is used to calibrate instruments and to assess aircraft performance, uniformly.

The four to know by heart
Pressure at mean sea level1013.25 hPa
Temperature at mean sea level15 °C
Density1.225 kg/m³
Water vapourNone
Good to know
0 ft36 090 ft65 600 ft−56.5 °C0 °C15 °CTemperatureTropopause, 11 km · 226.3 hPa−1.98 °C per 1 000 ftA constant −56.5 °C
Drawn to scale. 15 °C, less 1.98 °C for every 1 000 ft, comes to −56.5 °C at 36 090 ft.
Temperature lapse rate1.98 °C per 1 000 ft.
Pressure lapse rateAbout 1 hPa per 30 ft. How it is used is in ATM 005.
The tropopauseAt 36 090 ft (11 km), where the pressure is 226.3 hPa.
Above itA constant −56.5 °C, from 36 090 to 65 600 ft.

Density

Density here means air density: the mass per unit volume of the atmosphere, or the amount of molecules in a given volume of air. Move the sliders to see how each thing changes it.

Try
Pressure1 013 hPa
As pressure decreases, density decreases.No change
The ISA’s pressure at sea level
Temperature15 °C
As temperature increases, density decreases.No change
Moisture0% humidity
As moisture increases, density decreases.No change
AirWater vapourThe same volume every time
1.225kg/m³100% of the ISA’s 1.225

The ISA at mean sea level, or near enough.

Every box is the same volume. Fewer molecules in it, or lighter ones, is less mass: a lower density. Moisture has the smallest effect of the three, because even on a humid day water vapour is only a few per cent of the air.

Higher upDensity decreases with height, because the pressure does.
Not measured directlyIt is calculated from temperature, pressure and humidity. In the ISA it is 1.225 kg/m³.
Reduced air density results in
Less lift created
A less efficient rotor or propeller
Less performance from a piston engine
Less drag

The one effect that helps.

Density and the human body

Higher up, the air is thinner, so each breath holds less oxygen. The lack of oxygen causes hypoxia.

HypoxiaLight or gradual

Headache, fatigue, a feeling of euphoria, and nausea.

HypoxiaSevere or rapid

Changes in the level of consciousness, coma and death.

Time of useful consciousness

How long a person can still act usefully, at each level, once the oxygen runs short.

FL 150
30 min or more
FL 220
5 to 10 min
FL 280
2.5 to 3 min
FL 350
30 to 60 s
FL 450
9 to 15 s

The bars are on a log scale, so seconds and half hours fit on one line. The pale part of each bar is the range.

Pressure systems

How pressure is drawn on a weather map, and the shapes it makes there.

IsobarsLines drawn on a weather map connecting points of equal pressure. Generated from mean sea level pressure reports.
LLowTroughHHighRidgeEach line is an isobar
Drawn, not a real chart. A trough stretches out of a low, a ridge out of a high.
L
Low pressure area
H
High pressure area
Trough
An elongated area of low pressure.
Ridge
An elongated area of high pressure.
Col
An area of slack pressure between two highs and two lows, arranged crosswise.
LHHL Col slack pressure
Drawn, not a real chart. The isobars ring each high and low closely, and open out in the middle: the col.

Reading material

Meteorology

MET 002 The Atmosphere PowerPoint / PDF
EPN Group Training Document Meteorology, chapter 2 Reading list, course material
Ahrens, C. Donald, Meteorology Today: An Introduction to Weather, Climate, and the Environment, 8th edition, 2007 Reference, paid
WMO-No. 731, Guide to Meteorological Observing and Information Distribution Systems for Aviation Weather Services, 2014 edition Reference, free
European Union Aviation Safety Agency, Easy Access Rules for Air Traffic Management/Air Navigation Services (Regulation (EU) 2017/373), March 2025 Reference, free