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MeteorologyLesson 5 of 5

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MET 005

Atmospheric circulation

Wind

What wind is, and five ways it matters to the traffic.

Air in motionCaused by differences in pressure (MET 002).
Why it matters
  • Over the groundWind affects an aircraft’s motion over the ground.
  • Landing and take-offMade into the wind. Hazardous windshear, turbulence and crosswind.
  • VectoringThe differences between track and heading, and between airspeed and groundspeed.
  • Friction layersThe wind changes through the friction layers.
  • Route and levelRoute and level changes due to head- and tailwind, such as the jet stream.

Meteorological scales

Four scales, from the largest to the smallest.

GlobalTrade winds and global weather patterns.
SynopticCyclones, low- and high-pressure systems, fronts.
MesoscaleLand and sea breeze, föhn wind (MET 004), squall lines, individual thunderstorms.
MicroscaleLocal turbulence.

Global atmospheric circulation

The large-scale movement of air.

How heat is shared outThe means, together with the smaller oceanic circulation, by which thermal energy is distributed on the surface of the Earth.
What drives itDifferent amounts of solar radiation are absorbed at different latitudes (MET 003).
Surface heated from the sun
HLLHHeated surfaceRisingDescendingMoving horizontally, losing its heat1234
Warm air rises from the heated ground and leaves lower pressure at the surface. Cooled air sinks and builds higher pressure. Along the surface, the air flows from the high to the low.
  1. Air expands and starts risingBecause of heating at the surface.
  2. Lower pressure at the surfaceAn area with relatively lower pressure is created at the surface.
  3. It moves horizontally, then descendsAs the air stops rising, it moves horizontally, loses its heat and eventually descends again.
  4. Higher pressure at the surfaceAn area with relatively high pressure develops where it meets the surface.

Different surface heating

Why the sun heats some latitudes more than others, and the circulation that follows.

23.5°Perpendicularto the orbitOrbit (the ecliptic)EquatorAxisSunlight
The axis leans 23.5° from the perpendicular to the orbit, so the angle of the sunlight and the length of the day at each latitude change through the year.
Tilted 23.5°Earth’s axis is tilted 23.5° from the perpendicular to its orbit (the ecliptic).
SeasonsThis causes seasonal variations, depending on latitude.
Angle and daylightIt affects the angle of incidence and the duration of the day.
Global circulation of air reduces the heat difference between the equator and the poles.

The Coriolis effect

The apparent deflection, seen from the turning Earth, of anything moving freely over its surface.

SlowFastIf the discstood stillDeflectedto the rightOne rate of turningAir leaving the rim
A disc turning as the northern hemisphere turns, seen from above the pole: its rim moves fastest, as the ground does at the equator. Air leaving the rim keeps that speed, overtakes the slower surface, and bends to the right.
One rate of turningThe whole Earth turns at one rate. The ground moves faster at the equator than near the poles.
Air overtakes the groundAir moving away from the equator moves over ground that is moving more slowly, and so it overtakes the ground.
In the northern hemisphere, moving air is deflected to the right.

The three-cell model

The northern hemisphere in cross-section, from the North Pole to the equator.

Polar cellFerrel cellHadley cellTropopausePolar jetSubtropical jetHLHLNorth Pole60°N30°NEquator
Air rises at the equator and at 60°N, where the pressure at the surface is low, and sinks at 30°N and at the pole, where it is high.
Polar cellBetween the pole and 60°N. Along the ground the air flows from the pole to 60°N.
Ferrel cellBetween 60°N and 30°N. Along the ground the air flows from 30°N to 60°N.
Hadley cellBetween 30°N and the equator. Along the ground the air flows from 30°N to the equator.
The tropopauseRises from the pole towards the equator (MET 002). It breaks at about 60°N, where the polar jet is, and again at about 30°N, where the subtropical jet is.

Reading material

Meteorology

MET 005 Atmospheric Circulation PowerPoint / PDF
EPN Group Training Document Meteorology, chapter 5 Reading list, course material
Lankford, Terry T., Aviation Weather Handbook, 1st edition, 2000 Reference, paid
EUMeTrain, SatManu: Manual of Synoptic Satellite Meteorology (Conceptual Models) (viewed 20 September 2019) Reference, free
National Hurricane Center and Central Pacific Hurricane Center (viewed 20 September 2019) Reference, free
International Civil Aviation Organization, Annex 3, Meteorological Service for International Air Navigation, 21st edition, August 2025 Reference, paid