An air mass is a large body of air whose properties of temperature and humidity are similar in any horizontal direction. Air masses can cover hundreds of thousands of square miles. Air masses are formed when air stagnates for long periods of time over a uniform surface. The characteristic temperature and moisture of air masses are determined by the surface over which they form. An air mass acquires these attributes through heat and moisture exchanges with the surface.
Air masses are classified according to the temperature and moisture characteristics where they develop. Cold air masses originate in polar regions and are therefore called polar air masses. Warm air masses usually form in tropical or subtropical regions and are called tropical air masses. Moist air masses form over oceans and are referred to as maritime air masses. Dry air masses that form over land surfaces are called continental air masses.
In a typical year, weather changes resulting from the movement of air masses kill more people in the United States than all other weather phenomena combined. For instance, people from warmer winter climates (like Dallas, Texas) might be at greater risk from a cold air outbreak than people living in colder climates (like Madison). Conversely, Madisonians may be at greater risk to heat waves than residents of Dallas.
Socioeconomic status, cultural backgrounds and the size of the elderly population can also put a city’s residents at greater or lesser risk. For example, households with heaters in the north and air conditioners in the south may reflect socioeconomic status, whereas cultural backgrounds may influence the amount of time people spend outdoors.