Air masses are large bodies of air (over 1,000 miles in diameter) that possess similar

temperature and moisture characteristics throughout. An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days, or longer, it picks up the distinct temperature and humidity characteristics of that region.

The basic types of air masses are: maritime tropical (mT), continental tropical (cT), maritime polar (mP), continental polar (cP), and continental arctic (cA).

A front is the area along which two different air masses meet. At a front, the two air masses have different densities, based on temperature, and do not easily mix. One air mass is lifted above the other, creating a low pressure zone. If the lifted air is moist, there will be condensation and precipitation. Winds are common at a front. The greater the temperature difference between the two air masses, the stronger the winds will be. Fronts are the main cause of stormy weather.

Along a cold front, the denser, cold air pushes up the warm air, causing the air pressure to decrease. At the front, there will be a line of rain showers, snow showers, or thunderstorms with blustery winds. Along a warm front, a warm air mass slides over a cold air mass. When warm, less dense air moves over the colder, denser air, the atmosphere is relatively stable.