Geography · Ch 15 — Atmospheric Circulation and Weather Systems
Air Masses
Air Masses
An air mass is simply a very large volume of air that has taken on the character of the place where it has been sitting. When air stays over a uniform surface — a vast ocean or a huge flat plain — for a long enough time, it gradually picks up the temperature and moisture of that surface. The result is a body of air that is fairly consistent all the way across: little horizontal variation in temperature and moisture. That body of air is called an air mass.
The uniform surfaces over which these air masses form are known as source regions. Because the air inherits its properties from its source, air masses are classified according to where they come from. The book recognises five major source regions:
- Warm tropical and subtropical oceans
- Subtropical hot deserts
- Relatively cold high-latitude oceans
- Very cold, snow-covered continents in high latitudes
- Permanently ice-covered continents in the Arctic and Antarctica
From these five source regions, five types of air masses are recognised:
| Air Mass | Source Region |
|---|---|
| Maritime tropical (mT) | Warm tropical and subtropical oceans |
| Continental tropical (cT) | Subtropical hot deserts |
| Maritime polar (mP) | Relatively cold high-latitude oceans |
| Continental polar (cP) | Very cold, snow-covered continents in high latitudes |
| Continental arctic (cA) | Permanently ice-covered continents in the Arctic and Antarctica |
When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains. The air with distinctive characteristics in terms of temperature and humidity is called an airmass -- it is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions. …