How do forecasters know what the weather will be like tomorrow and the next day and the day after that? Like modern day fortune tellers, they predict the future and get it right most of the time. But theseprognosticators don't read tea leaves or gaze into crystal balls. They use their knowledge of air masses.
An air mass is a large body of air having relatively uniform temperature and humidity. If air stays in one place long enough, it takes on the characteristics of the surface below. For example, air over the Gulf of Mexico becomes warm and wet while air gathered over the northern plains of Canada becomes cold and dry.
The boundary between two air masses is known as a front. When air masses move, they collide with other air masses usually causing some kind of precipitation. A cold front occurs when cold air replaces warm air. A warm front is when warm air replaces cold air.
There are five types of air masses which determine the weather in the United States. They are identified by where they form and the temperature and humidity of the air. Some air masses affect certain parts of the country more than others.
Continental polar air masses (designated cP) form over northern Canada. During winter months, these cold, dry air masses sweep down through the United States sometimes bringing cold all the way to the Gulf Coast. During the summer, they bring clear skies and pleasant temperatures to the northern U.S.
Continental tropical air masses (designated cT) form over the warm, dry deserts of the southwest. These form mostly during the summer, and can bring record heat waves to the Plains and Mississippi Valley. As they move eastward, they usually pick up moisture and become cooler and less dry.
Maritime tropical air masses (designated mT) usually form over the southern Atlantic ocean or the Gulf of Mexico. They can form over the southern Pacific as well. These bring warm air with heavy amounts of moisture. The southeastern United States gets its reputation for hot and humid summer days from these air masses, although they can form anytime throughout the year.
Maritime Polar air masses (designated mP) form in the Pacific off the coasts of Alaska and western Canada and in the Atlantic off the coasts of Maine and eastern Canada. They are not as cold as continental polar air masses. Forming year round, they give Seattle its rainy disposition and New England its legendary fog.
One final type of air mass, continental Arctic (designated cA), forms in the winter over the frozen ice sheets of the Arctic Ocean. These cold, dry blasts of air can swoop down across Canada and the United States bringing rapidly falling and sometimes recod-breaking temperatures.
Understanding these five types of air masses, how they move and how they interact is the key to solving the mystery of weather forecasting. Look for the air masses and you’ll never look at a weather map the same way again.