Air masses carry with them the characteristics of whatever part of the country in which they form. These systems form over specific areas, usually where the land is relatively level and the wind is calm. Known as source regions, these areas may produce air masses that are cold, humid, dry, or hot, and the longer they remain over these areas, the more properties it will take on. As they move horizontally across the land or water, they carry those traits with them to other areas. When two of these masses meet, a front is formed, producing most of the more impressive weather conditions.
The air masses that affect the U.S. are continental polar, continental arctic, continental tropical, maritime polar, and maritime tropical. Just as their names imply, continental air masses form over land, and maritime, over water. Continental and maritime air masses may both be cold, as with the polar and arctic, or hot, such as the continental or maritime tropical, however, the continental are drier than the maritime air masses.
Continental polar or continental arctic air masses are those that we hear about in the winter months as cold, dry air moves down into the continental U.S. from Canada and Alaska. Moving south, they affect areas east of the Rockies as they make their way eastward. These generally have high pressure, very cold temperatures, and low dew points.
Maritime polar are also cool, but they are also moist and unstable. This air mass moves over water, either westward over the Pacific, or southwestward over the Atlantic. Both of these situations produce an air mass that is traveling over water, and therefore, collecting moisture as it approaches land. Air masses that arrive from the North Atlantic are normally cooler and drier than those that enter from the Pacific.
Maritime tropical air masses are the culprits that produce sometimes violent storms in the Great Plains and across the Midwest in the spring and summer. These moist, warm and normally unstable air masses begin in the subtropical regions of the Pacific Ocean, where they travel over water for a long period of time, picking up steam as they go. Originating for the most part in the Gulf of Mexico, they produce low clouds and unstable conditions as they make their way northward.
Continental tropical air masses, that have their origins in northern Mexico, are hot and dry. These produce clear skies and very little rain, and if one of these air masses stalls out over a particular area, they may produce a long period of drought.
Air masses may change as they move across the country. An arctic air mass, after traveling over water for a period of time, may turn into a maritime polar air mass. Both are cold, but the addition of moisture changes the classification.
While we can’t see them, everyone is aware of air masses, and the passage of fronts. Extreme temperature variations over a matter of hours, sudden storms, and periods of wind, are all indications that these air masses are passing through or converging.