An air mass is a vast pool of air having similar temperature and moisture characteristics over its horizontal extent. An air mass occupies thousands of square miles of the Earth's surface. Air masses are born in a source region where they take on their characteristic temperature and moisture content. Source regions are often regions of low relief and calm wind that prevent turbulent mixing and allow the air to take on the conditions of the surface over which it forms. Radiation and vertical mixing of heat yield an equilibrium between the conditions at the source region and the properties of the overlying air mass after a period of 3 to 5 days. Areas dominated by high pressure serve as good source regions where subsidence pushes the air toward the surface. High pressure also enables the air to move outward from the source region.
Atmospheric scientists have created definite temperature and humidity criteria to classify each air mass. We’ll classify them based on their general conditions, e.g. warm and wet, cold and dry. The latitude of the source region fundamentally determines the temperature of an air mass. Arctic air masses form between 60o and 90o north latitude. Arctic air masses are characterized as being extremely cold air masses. Polar air masses form between 40o and 60o north or south latitude and are cold air masses but warmer than the higher latitude arctic air mass. Warm tropical air masses are found between 15o and 35o north and south latitude. The exceedingly warm equatorial air masses form near the equator. The type of surface over which air masses form also determines their humidity characteristics. Maritime (oceanic) air masses are typically moist, whereas those forming over the continents are usually dry. However, humidity is also determined by temperature so cooler maritime polar air masses are drier than warm maritime tropical air masses.
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