Tomorrow's weather usually arrives from somewhere else, already made. This lesson is about the bodies of air that carry it: what an air mass is, where the great ones are manufactured, the letter codes that name them on a surface chart, the character each brings to Europe, and how the journey from source to destination rewrites that character, with the stability rules of the thermodynamics module doing the rewriting.
What an air mass is
An air mass is a large body of air, a thousand kilometres and more across, whose temperature and humidity are nearly uniform horizontally at any given level. Air acquires that uniformity by sitting still: a source region must offer an extensive, uniform surface (ice cap, ocean, desert) and stagnant or light-wind conditions, which in practice means the slow-turning air of a large anticyclone, so that the air rests on the surface long enough to take on its temperature and moisture through the layers near the ground. The subsiding, settled character of highs is met again in the pressure-systems module; here it is enough that they are the atmosphere's factories.
Source regions and the classification
The first sorting is by source region, from poleward to equatorward, into four types:
Arctic (A) — the polar ice caps: intensely cold, dry at source.
Polar (P) — the high-latitude belts around them: cold.
Tropical (T) — the subtropical highs: warm.
Equatorial (E) — the equatorial oceans and rainforests: hot and very moist.
The second sorting is by temperature and humidity at source:
Maritime — born over an ocean: moist.
Continental — born over land: dry.
And each is cold or warm according to its birthplace, arctic cold to equatorial hot. Two words per air mass, and most of its biography is already written.
The letter codes of the chart
On a surface weather chart the description compresses to three letters, and the examination uses this exact scheme:
First letter — humidity: continental (c) or maritime (m).
Second letter — type: arctic (A), polar (P), tropical (T) or equatorial (E).
Third letter — temperature: cold (c) or warm (w), relative to the surface the air is currently crossing, not to any fixed thermometer reading.
The third letter is the working one, because it encodes stability:
mPc — maritime polar cold: moist polar air colder than the surface beneath it, heated from below, therefore unstable, building cumulus and showers.
mTw — maritime tropical warm: moist tropical air warmer than the surface, cooled from below, therefore stable, spreading stratus and fog.
Read the third letter and the stability rules of the thermodynamics module tell you the weather before the satellite picture loads.