AIR MASSES AND FRONTS

AIR MASSES AND FRONTS • Air Masses • Six Kinds of North American Air Masses • Characteristics of Each Air Mass • Kinds of Fronts • Front Interactions • Principal Storm Tracks

AIR MASSES • The Second Law of Thermodynamics predicts that any undisturbed system will spontaneously tend toward maximum sameness, a uniform blending. • If a large body of air sits over the polar ice cap, the air will eventually become a uniform body of cold, dry air; Polar Continental Characteristics. This is an air mass. • If a large body of air sits over a tropical ocean, the air will eventually a uniform body of warm, moist air; Maritime Tropical Characteristics. This is also an air mass. • As the Earth turns and winds blow, air masses don’t just sit still, they eventually collide and interact. • When two air masses collide, the zone where they meet is called a front.

SIX KINDS OF AIR MASSES Maritime Polar Continental Polar Maritime Tropical Continental Tropical Maritime masses can be either Atlantic or Pacific masses

AIR MASSES AND THEIR CHARACTERISTICS

FRONTS • When two air masses collide, the zone where they meet is called a front. The movement of air along a weather front is fundamental to our understanding of weather. • Whenever warm air comes in contact with cold air, the warm air, being less dense than the cold air, rises. • This leads to a zone of low barometric pressure, which frequently gives rise to cloud formation and precipitation. • The character of the low pressure zone and the weather that results depend on the relative velocities of the two parcels of air.

WARM FRONT If a warm air mass overtakes a mass of cold air, the warm air rises, but AS IT RISES IT CONTINUES TO MOVE FORWARD. The net result is that it flows up and over the cold air, pushing the cold air into a wedge. The warm air extends over the cold air for a distance of several hundred kilometers. This rising air leads to a storm system that is often 500 to 600 km wide and is characterized by low pressure, stratus or nimbostratus clouds, and persistent rain or snow.

COLD FRONT A cold front occurs along a zone where warm air and cold air masses come in contact. In this case, the cold air overtakes the warm air. During this collision, the faster moving cold air cannot slide over the warm air because it is more dense. The cold air is distorted into a blunt wedge or bulge as it pushes under the warm air. The contact zone of rising air and precipitation is quite narrow. The warm air rises quite rapidly, and the result is a narrow band of violent squalls only 50 to 150 km wide, that may contain downpours, thunderstorms, and even tornadoes.

OCCLUDED FRONT This front occurs when two fronts collide, and trap another air mass between them. In this example, a parcel of warm air is trapped between two parcels of cold air and is lifted completely off the ground. This system combines the narrow zone of violent winds and precipitation of the cold front with wider and more gentle precipitation zone of the warm front. The net result is a large zone of inclement weather.

PRINCIPAL STORM TRACKS

STORM TRACK OBSERVATIONS • All storms tend to converge towards the Northeast. • Although many storm tracks appear to begin just east of the Rockies, in reality they began over the Pacific Ocean. Traveling through the mountains, they lose their identities temporarily and redevelop on the Great Plains just east of the Rockies. • Storms that are formed in the south tend to have much more precipitation than those that form in the north because southern storms are closer to sources of moisture rich, maritime tropical air. • Storms are moved by winds high in the middle and upper troposphere. • Keep in mind that there are two motions associated with a storm: • within the storm there is a spinning, cyclonic movement counterclockwise around the cyclone center; • the storm center, and the storm itself moves towards the Northeast

AIR MASSES AND FRONTS