The temperature and humidity of a region on a given day depend largely on the characteristics of the air mass that lies above it. An air mass forms when air sits over a region for several days or longer
and acquires the distinctive temperature and humidity of that region. This happens because heat and moisture are transferred between the ground surface and the air above it until the air has the same characteristics as the ground. For example, air masses that form over oceans are moister than those that form over continents; those that form over polar regions are colder than those that form over the tropics.
When an air mass leaves the place where it formed, it brings its distinctive temperature and humidity characteristics to its new location. Stormy weather may result if the characteristics of the air mass are different from those of the new region. When a cold air mass moves over warm ground, for example, the bottom of the air mass is heated. The warmed air then rises, leading to clouds, rain, and possibly thunderstorms. When a warm air mass travels over cold ground, the bottom of the mass cools and forms a temperature inversion, in which air temperature increases with altitude. Inversions trap air, including pollutants, over a region because the cold air near the ground cannot rise into the warm air above it.
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