Air Masses — Reading Comprehension Lesson

Reading Passage

Air Masses

An air mass is a large body of air — often thousands of miles across — that has relatively uniform temperature and moisture throughout. Air masses form when air stays over a region for an extended period and takes on the characteristics of the surface below it. An air mass that forms over the cold land of northern Canada becomes cold and dry. One that forms over the warm waters of the Gulf of Mexico becomes warm and moist. Meteorologists classify air masses by their source region: continental air masses form over land and tend to be dry; maritime air masses form over ocean and tend to be moist.

Air masses are further classified by latitude: polar air masses form at high latitudes and are cold; tropical air masses form at low latitudes and are warm. Combining these two characteristics gives four main types: continental polar (cold and dry), maritime polar (cold and moist), continental tropical (warm and dry), and maritime tropical (warm and moist). The weather in much of the United States is strongly influenced by the interaction of these air mass types. Much of the country’s severe weather — thunderstorms, tornadoes, blizzards — occurs when different types of air masses collide.

When air masses meet, they do not mix easily because of their different densities. The boundary between two air masses is called a front. A cold front forms when a cold air mass pushes under a warm air mass, forcing the warm air to rise rapidly. This rapid lifting produces towering cumulonimbus clouds, heavy rain or hail, and strong thunderstorms, often followed by a sharp drop in temperature and a shift in wind direction. Cold fronts typically move faster than warm fronts and produce more intense but shorter-lived storms.

A warm front forms when a warm air mass advances and slides up over a retreating cold air mass. The gradual lifting of warm air over the cold produces a broad sequence of clouds that can extend hundreds of miles ahead of the front — first high cirrus clouds, then altostratus, then nimbostratus producing steady rain or snow for hours or days. After a warm front passes, temperatures rise and winds shift. A stationary front occurs when neither air mass is advancing, bringing prolonged cloudy and rainy conditions. Understanding air masses and fronts is the foundation of weather forecasting.

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