An air mass is a huge body of air, often over a thousand kilometers across, that has taken on the temperature and moisture of the region where it sat. Winds carry air masses away from home, and where two different ones meet, the boundary is a front.
| Air mass | Formed over | Character | Example for North America |
|---|---|---|---|
| Continental polar (cP) | Cold land | Cold and dry | Northern Canada |
| Maritime polar (mP) | Cold ocean | Cool and moist | North Pacific, North Atlantic |
| Maritime tropical (mT) | Warm ocean | Warm and humid | Gulf of Mexico |
| Continental tropical (cT) | Hot land | Hot and dry | Deserts of Mexico and the Southwest |
Key idea
Where two air masses meet, the denser, colder air stays low and the warmer air is lifted over it. Rising air cools, its water vapor condenses, and clouds and precipitation form along the front.
Why it works
Cold air is denser, so it hugs the ground like a wedge. At a cold front, cold air plows under warm air and shoves it up steeply: tall clouds, short heavy showers or thunderstorms, then cooler, drier air. At a warm front, warm air slides gently up over the retreating cold air: wide sheets of layered clouds and long, steady rain, then warmer, more humid air. The lifting is the same condensation step of the water cycle.
Worked example: Name the air mass
An air mass forms over the Gulf of Mexico. What is it like?
- Over an ocean: maritime, so moist.
- In the warm south: tropical, so warm.
- Maritime tropical (mT): warm and humid.
Answer: Warm and humid (mT)
Worked example: After the front
A line of thunderstorms passes, the wind shifts, and the next day is cooler and drier. What kind of front passed?
- Short, heavy storms point to steep lifting.
- Cooler, drier air behind it means cold air moved in.
- A cold front.
Answer: A cold front
Common mistake
Thinking warm and cold air mix right away at a front. The denser cold air stays low and wedges under the warm air, lifting it. That lifting, not mixing, is what makes the clouds. Check: fronts are drawn as sharp lines on weather maps because the two air masses stay separate for days.