An air mass is an enormous volume of the lower atmosphere — frequently more than 1,600 kilometers across and several kilometers deep — in which temperature and moisture content are nearly uniform in the horizontal direction. Uniformity on that scale does not happen by accident. It requires the air to remain nearly stationary over a broad region of unchanging surface conditions for several days, long enough for conduction, radiation, and evaporation to imprint the properties of the underlying surface onto the atmosphere above it. The region where that imprinting occurs is the source region.

Two conditions make a productive source region. First, the surface itself must be uniform: an unbroken ice sheet, a continental interior of similar soil and vegetation, or an open expanse of ocean. Second, the large-scale circulation must be sluggish, which is why semi-permanent high-pressure systems are the classic incubators of air masses. Air descending inside a high suppresses mixing with the surroundings and gives the lower atmosphere time to equilibrate with the surface below. The middle latitudes, dominated by fast westerly winds and an endless parade of storms, make poor source regions; the polar and subtropical latitudes make excellent ones.

Meteorologists label air masses with a two-letter code. The lowercase first letter describes moisture and reflects the surface: c for continental (formed over land, therefore dry) and m for maritime (formed over water, therefore moist). The uppercase second letter describes temperature and reflects latitude: A for Arctic, P for Polar, and T for Tropical. Combining the two yields the five air masses that govern weather across the United States: cA, cP, mP, cT, and mT.

Because temperature and humidity together determine density, the code also predicts behavior. A cA air mass is cold and dry, so its molecules are packed tightly and it hugs the ground and spreads outward. An mT air mass is warm and humid, and because water vapor molecules are lighter than the nitrogen and oxygen they displace, humid air is less dense than dry air at the same temperature. When these two meet, the denser air always wedges beneath the lighter air — the process you will study in Lab 3.

Reading 2 — Air Masses Change as They Travel

An air mass does not stay the same once it leaves its source region. As it moves across terrain with different temperature and moisture characteristics, it undergoes modification. Because heat and water vapor are exchanged at the ground, modification always begins in the lowest layer and works upward, which means the bottom of a traveling air mass is always the first part to change.

The direction of the change depends on a simple comparison: is the surface warmer or colder than the air above it? When an air mass crosses a warmer surface it is heated from below. The lowest layer warms, becomes less dense, and rises, so the air mass becomes unstable and convection develops. When an air mass crosses a colder surface it is cooled from below. The lowest layer becomes denser and settles, so the air mass becomes stable and vertical motion is suppressed.

Two familiar New York examples show both outcomes. In November a cP air mass streams south out of Canada at roughly −10 °C and crosses Lake Ontario, whose water is still near 8 °C. The lake is dramatically warmer than the air, so heat and water vapor pour upward. The modified air becomes unstable, rises, condenses, and unloads lake-effect snow on the downwind shore — which is why Syracuse can be buried while Albany, only 200 kilometers away, stays clear. The opposite happens in April, when warm mT air moves north over the still-frigid ocean south of Long Island. That air is chilled from below until it reaches its dew point, and dense advection fog forms along the coast.

Human impact. Modification is not an academic detail. Communities downwind of the Great Lakes budget millions of dollars each winter for snow removal, and the length of that snow season depends directly on how long the lakes stay unfrozen. Coastal airports on Long Island lose flight hours every spring to advection fog produced by exactly the process described above. Forecasters who understand modification can warn these communities days ahead; forecasters who treat air masses as unchanging cannot.