Air masses

Where do air masses come from, and how do you read their names?

What an air mass is

Four cards, then decode them yourself.

1. One air mass An air mass is a very large body of air in which temperature and humidity are nearly the same, side to side and up and down, over thousands of square kilometres.
2. Source regions It forms where air sits still for a long time over a uniform surface and takes on its temperature and moisture: the light winds of the big highs over the poles and the subtropics. Over water it becomes moist, over land dry; from cold places cold, from hot places warm.
3. Two letters Polar (P) = cold, tropical (T) = warm. Continental (c) = dry, maritime (m) = moist. So cP is cold and dry, mP cold and moist, cT warm and dry, mT warm and moist. Add arctic (A, extremely cold and dry) and equatorial maritime (Em, hot and very humid).
4. Modified on the way As an air mass moves it carries its properties with it, but it is changed by what it passes over: a cold air mass over a warm sea is heated from below and turns unstable; a warm one over cold land is cooled from below and turns stable, with fog and low cloud.

Memory hook: Polar = cold, tropical = warm; continental = dry, maritime = moist.

Takeaway: An air mass is a huge body of air, thousands of km² across, with nearly uniform temperature and moisture. It takes its character from its source region (the poles and the subtropical highs, where air stagnates) and changes as it travels. The name gives it away: P cold, T warm, c dry, m moist.

What is each air mass like?

Decode the air mass

Tap an air mass, then its character.

• Polar continental (cP)
• Polar maritime (mP)
• Tropical continental (cT)
• Tropical maritime (mT)
• Arctic (A)
• Equatorial maritime (Em), India's SW monsoon
• Air that formed at sea in high latitudes
• Air from a subtropical desert

Takeaway: cP cold and dry, mP cold and moist, cT warm and dry, mT warm and moist, A extremely cold and dry, Em hot and very humid.