An air mass consists of a vast quantity of air that possesses a common temperature and moisture content throughout its reach. Air masses can form in either hot or cold regions, over large land masses or water bodies. They exhibit the features of their area of origin; for instance, an air mass born over the Caribbean Sea would likely be warm and moist. Air masses migrate throughout the planet, bringing with them weather conditions characteristic of their source region. Meteorologists generally classify air masses into four main categories on the basis of temperature and humidity.

Maritime Tropical

Maritime tropical air masses originate over oceans, within 25 degrees latitude of the equator. They are associated with warm, humid conditions and high dew points. Clouds often accompany maritime tropical air masses, ushering in the potential for rain and storms. In fact, most of the thunderstorms that occur in the U.S. develop in the presence of maritime tropical air masses. Maritime tropical air masses influence the U.S. more during summer than during winter, and exert the greatest influence in the Southeastern states.

Continental Tropical

Continental tropical air masses also form within 25 degrees latitude of the equator, but unlike maritime tropical air masses, they form over land instead of the sea. Continental tropical air masses typically spell dry weather, with cloudless skies, low humidity and negligible precipitation. Daytime temperatures become quite sweltering, but the mercury is apt to drop off sharply after sunset, as the cloudless skies promote the dispersion of heat into the atmosphere. Continental tropical air masses normally only influence weather in the U.S. in the summertime, and are most common in the Southwestern states.

Maritime Polar

Maritime polar air masses typically form over bodies of water north of 60 degrees latitude in the northern hemisphere and south of 60 degrees latitude in the southern hemisphere. Maritime polar air masses produce cool, damp conditions. Considerable cloudiness, including fog, often accompanies maritime polar air masses. Precipitation is also common. Depending on the time of year, precipitation may come in the form of rain or snow and can be substantial. In the U.S., maritime polar air masses are usually confined to the Pacific Northwest, although they occasionally affect New England.

Continental Polar

Like maritime polar air masses, continental polar air masses originate at high latitudes, but unlike maritime polar air masses, continental polar air masses form over land. Continental polar air masses bring about dry and cool to cold weather. Humidity and dew points are low, and any precipitation that falls tends to be light, due to the high pressure that typically accompanies these air masses. Some meteorologists recognize a subtype of continental polar air masses, which they refer to as Arctic or continental Arctic. Basically, such air masses are colder, harsher versions of the continental polar variety. In the U.S., continental polar and Arctic air masses most often affect the eastern two-thirds of the country, albeit with diminishing intensity further south.