An air mass is a huge body of air with similar temperature and humidity characteristics. Temperatures and humidities vary gradually with latitude, longitude, and altitude throughout the air mass. The air masses obtain their characteristic temperature and humidity by sitting at a location on earth for a period of time. This location is the source region. Air masses are NAMED by their temperature and humidity characteristics. The humidity characteristic is derived from the type of surface over which the air mass sits. The two main surface differences on earth are LAND and WATER. An air mass that sits over a water surface will have experienced a significant amount of evaporation, so that it will be considered humid or moist. It will be called a MARITIME air mass. Maritime air masses are designated by the small letter m. An air mass sitting over land will have experienced limited evaporation, so it will be considered to be dry. It will be called a CONTINENTAL air mass which is designated by the small letter c. In the name of an air mass, this humidity description is given first.

The second characteristic, that of temperature, is derived from the latitude of origin, since temperatures generally decrease poleward. Air masses originating near the equator are EQUATORIAL, designated by the capital letter E. These are considered hot air masses. At a higher latitude are the TROPICAL air masses, designated by the capital letter T. These are considered warm air masses. At still higher latitudes are the POLAR air masses, designated by the capital letter P. These are considered cool or cold, depending on the season. The highest latitude of origin is called the ARCTIC (southern hemisphere, Antarctic or AA), designated by the capital letter A. This air mass is considered very cold.

An air mass, then, is named by the combination of its humidity and temperature designation. With two terms for the humidity and four terms for the temperature, eight names are possible. These are: mE, mT, mP, mA, cE, cT, cP, cA (maritime equatorial, maritime tropical, maritime polar, maritime arctic, continental equatorial, continental tropical, continental polar, continental arctic). Actually ONLY SIX different air masses exist. Since the Intertropical Convergence Zone is noted for heavy precipitation, hot air can hold much moisture, and the equator does not have a large land area, equatorial air is not dry. This means the cE air mass is not found; it is mE air, moist and hot. The very cold high latitude air does not hold much moisture and does not have heavy evaporation or precipitation, so mA air is also not found. It is cA or very cold and dry.

name of air massdescription
mE maritime equatorial moist and hot
mT maritime tropical moist and warm
cT continental tropical dry and warm
mP maritime polar moist and cold
cP continental polar dry and cool or dry and cold
cA continental arctic dry and very cold

The world source regions are shown on the map below.
world air mass source regions

Air mass weather results from the dominance of an air mass. They are best associated with high pressure and anticyclones. Air mass weather means little variation in the temperature and humidity that will be experienced. A warm moist air mass may experience daytime surface heating, convection, and late afternoon thunderstorms. Night time cooling may result in heat loss and fog. The weather will generally be described by the characteristics of the dominating air mass.