14. Air masses are large-scale (regional) air parcels that have fairly consistent temperature and humidity throughout. They are responsible for most weather on a day-to-day basis.

Air masses are formed over large, continental sized areas with stable conditions and subsiding air. The nature of these source regions dictates the temperature and humidity characteristics of the air masses that form there (Fig. 1; Tbl. 1).
 

Figure 1. Air mass source regions for North America. Air masses originate in areas of stable, subsiding air. The frequency of frontal activity in the mid-latitudes prohibits air mass formation over the United States. As a result, our air masses come to us from other regions at higher and lower latitudes.

In terms of humidity, there are continental (c) source regions which are dry, versus maritime (m) source regions which are moist. Source region temperatures range from tropical (T) which is warm, to polar (P) which is cold, to arctic (A) which is very cold. These source region characteristics, when combined, provide for five different types of air masses. Note that there can be no mA air mass, as no ocean can remain ice free in arctic temperatures. In addition, very cold air can carry very little moisture. Hence a moist, very cold air mass is a contradiction of terms.
 
 

Table 1. Air mass classification based on humidity and temperature characteristics of the source region. 

	T = tropical (warm)	P = Polar (cold)	A = Arctic (very cold)
c = continental (dry)	cT (warm and dry)	cP (cold and dry)	cA (very cold and dry)
m = maritime (moist)	mT (warm and moist)	mP (warm and dry)	---

Air masses transport their characteristics from place to place in response to strong upper-air winds, such as the jet stream. As air masses move, they cause the weather to change and slowly change their own character as well. Therefore, our weather is a product of the character of the air mass(es) immediately around us.