Weather forecasters label giant blobs of air with a three-letter code
A blast of summer heat from the American desert is cT; a frigid slab from northern Siberia in winter is cA. Meteorologists sort air masses, bodies of air spanning hundreds or thousands of square miles, by dampness, birthplace and stability, and most dramatic weather happens where two of them collide.
An air mass takes on the temperature and moisture of the ground or sea beneath it. The most widely used scheme, the Bergeron classification, starts with a lowercase letter for humidity: c for dry continental air, m for moist maritime air. A capital letter gives the source: T for tropical, P for polar, A for arctic or antarctic, E for equatorial, M for monsoon, and S for air that has dried and warmed while sinking. An optional third letter, k or w, notes whether the mass is colder or warmer than the surface it is crossing, so polar air sliding over the Gulf Stream becomes cPk.
The notation can tell a story. A tick mark shows that an incoming mass is colder than the similar one it replaces, as in mPk followed by mPk'. A hyphen records change, so arctic air pushing out over the Gulf of Alaska might be written cA-mPk. A slash shows stacking, as when moist air from the Gulf of Mexico rides up over cold polar air across the central United States, written mT/cP.
The Sahara is the main factory for scorching, bone-dry continental tropical air, with Arabia, inland Australia and the south-western US deserts as smaller sources. North America's cold, dry continental polar air forms over interior Canada, while its humid tropical air comes from the Caribbean, the southern Gulf of Mexico and the Atlantic near the Bahamas. Arctic air, chilled over snow and ice, is colder than polar air but can be shallow in summer and change quickly as it heads south.
Fronts are the seams between masses of different density. Cold fronts, pushed by denser air behind them, usually outrun warm ones and can bring narrow lines of thunderstorms; warm fronts tend to arrive with steady rain and fog. Skies often clear fast once a front passes, and the wind always shifts. When the contrast fades, a stalled front can dwindle into a shearline, mostly over open ocean.
Source: Air mass
