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Air Masses

Air Masses are extensive bodies of air which have relatively uniform distributions of temperature and moisture. There are seven types of air masses that provide the major influence on the North American climate. The Continental Arctic (cA) air mass is characterized as cold and dry. The Continental Polar (cP) cool and dry and the Maritime Polar (mP) cool and moist. The boundary between two distinct airmasses is known as a front. If the airmass is warmer than the surface it passes over, it becomes more stable as it is cooled from below. On the other hand, a cold airmass moving over a warm surface is relatively unstable as it is being warmed from below. The unequal heating the earth receives in relation to the poles and the equator causes large scale movement of these air masses known as the atmospheric circulation. The wind flows due to pressure differences in the atmosphere towards areas of low pressure and away from areas of high pressure. (Remember that warm air rises!) The earth’s rotation causes the air to appear to be deflected to the right in the Northern Hemisphere. This deflection is called the Coriolis force. Three-Cell Theory postulates a pile up of cooled air at 30 degrees of latitude, outflow of this air to the north and south produces NE trade winds and SW westerlies. Westerlies meet cold polar air at 60 degrees of latitude. When two distinct air masses meet, the boundary zone between them is called a front. Across this zone, temperature, humidity and wind often change rapidly over short distances. This can mean significant changes to the flying conditions. When a cold airmass overtakes a warm airmass the cold air pushes under the warm air and forms a cold front. Warm air rising over and replacing cold air forms a warm front. If the boundary between air masses doesn’t move a stationary front is formed. Temperature is the most easily recognized change at frontal passage followed by changes in the wind and pressure. When flying across a front the wind will always veer or change direction clockwise in the Northern Hemisphere so that one will have to correct your heading to the right to compensate for the wind shift. More importantly the most dramatic changes in the weather are seen at the fronts. These changes are affected by available moisture, the stability of the air, the speed of the front, and temperature and moisture differences between the two air masses. The slope of the front is also a factor. A shallow slope usually promotes wide areas of cloud and precipitation. Steep slopes are usually characterized by narrow bands of cloud and showers. If moist and unstable is lifted, thunderstorms can be present.