In Earth Science you met air masses, huge bodies of air with the temperature and moisture of the place where they formed. With stability in hand, you can now see how an air mass changes as it travels, and why the Great Lakes bury some towns in snow.
Key idea
Air masses are named by their source: c (continental, dry) or m (maritime, humid), then P (polar, cold), T (tropical, warm) or A (arctic, very cold). Over North America: cP and cA from Canada and the Arctic, mP from the North Pacific and North Atlantic, mT from the Gulf of Mexico and the warm Atlantic, and cT from the deserts of Mexico and the Southwest in summer. As an air mass moves over new ground or water, it is modified from below.
Why it works
Air is heated or cooled mostly from the ground up. Cold air moving over warmer water or land is warmed at the bottom, so its lowest layer becomes warm compared with the air above: unstable. Bubbles rise, carrying heat and moisture up, and clouds and showers form. Warm air moving over cold ground is chilled at the bottom: a stable layer forms, often with fog or low stratus. In lake-effect snow, frigid cP air crosses a lake that hasn't frozen yet. The lake heats and moistens the bottom of the air, the air becomes unstable, and bands of heavy snow form on the downwind shore.
Worked example: Will the lake make snow?
In December a lake's surface water is 6 °C, and the air 1.5 km up (at about 850 hPa) is −10 °C. Forecasters use a rule of thumb: lake-effect snow is likely when the lake is at least 13 °C warmer than the air at 850 hPa. Is it likely?
- Difference: $6 - (-10) = 16$ °C.
- 16 °C is more than 13 °C, so the air over the lake is unstable enough.
- In fact, 16 °C over about 1.5 km is about 10.7 °C per km, faster than the dry rate (9.8): absolutely unstable.
Answer: Yes, likely
Worked example: Which shore?
A cold northwest wind blows across Lake Ontario. Which shore gets the lake-effect snow?
- The air picks up heat and moisture as it crosses the lake.
- It drops the snow when it reaches land on the downwind side.
- A northwest wind blows toward the southeast: the southeast shore.
Answer: The southeast shore
Common mistake
Thinking an air mass keeps its properties forever. Crossing warm water, cold ground, mountains or deserts changes its temperature and moisture from below. Arctic air reaching Florida is much warmer than when it left Canada; Pacific air that crosses the Rockies arrives on the Plains much drier. Check: ask what the ground or water under the air is doing to its lowest layer.