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AIR MASSES AND FRONTS Earth Science Mrs. Nelson, MMS History Scientist thought that storms were caused by low air pressure up until the early 1900. This was based on the fact that storms always seemed to occur in low-pressure areas. Cause of storms In the early 20th century Vilhelm Bjerknes, a Norwegian meteorologist and a group of his colleagues concluded that storms are caused by the collision of large air masses that differ from one another in temperature and humidity. Storms occur in low pressure areas, but low pressure is not their cause. Low Pressure Low pressure, like storms, are a result of the collision of different air masses. This finding is now accepted as one of the most important principles of modern meteorology, and it is the basic principle underlying weather forecasting today. Air Masses and Fronts What are major types of air masses in North America, and how do they move? What are the three main types of fronts? What types of weather is associated with cyclones and anticyclones? Air Masses A huge body of air that has similar temperature, humidity, and air pressure at any given height is an air mass. Scientist classify air masses according to temperature and humidity. Four major types of air masses influence the weather in North America. Four Major Types of Air Masses Maritime tropical Continental tropical Maritime polar Continental polar Temperature affects air pressure Cold, dense air has a higher pressure than warm, less dense air, which has a lower pressure. Tropical, or warm air masses form in the tropics and have low air pressure. Polar, or cold, air masses form north of 50 degrees south latitude. Polar air masses have high air pressure. Air masses are named according to the part of the Earth’s surface over which they are formed. Maritime tropical air masses (mT) are moist and warm. They form over oceans. They are very humid and have low pressure. Continental tropical air masses (cT) are warm and dry. They form over land. They are drier than maritime air masses. Air Masses Maritime polar (mP) air masses are moist and cool to cold. Maritime air masses form over oceans and are humid. Continental polar (cP) air masses are dry and cold. They have low humidity. In summer, the air mass is milder. Storms may occur when continental polar air masses move south and collide with maritime tropical air masses moving north.