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Air Masses & Fronts High and Low Pressure Areas • High pressure causes air to sink • Usually results in several days of clear sunny skies • Air rises in low pressure areas and forms water droplets • Usually results in rain and storms What are the four types of air masses? Maritime Tropical Maritime Polar Continental Tropical Continental Polar Maritime Tropical • Warm humid air masses from oceans near the tropics. • They form over the Gulf of Mexico and the Atlantic Ocean. • In the summer time maritime tropical usually bring hot humid weather. • In winter, a humid air mass can bring heavy rain or snow. Maritime Polar Cool humid air masses form over the icy cold North Pacific and North Atlantic oceans. The air masses affect the west coast more than the east coast. In the summer they often bring rain, fog, and cool temperatures to the west coast. Continental Tropical Hot dry air masses form only in the summer over dry areas of southwestern and northern Mexico. Cover a smaller area than other air masses. Bring hot, dry weather to the south. Continental Polar Form over central and northern Canada and Alaska. Bring cool and cold air. In winter they bring clear, cool, and dry air to most of northern America. In the summer, storms may occur when continental air masses move south and meet maritime tropical that move north. How do air masses move? The prevailing westerlies are the major wind belts in the United States. Prevailing westerlies push air masses from west to east. Fronts A front is the boundary separating air masses of different densities Fronts extend both vertically and horizontally in the atmosphere Fronts: FourTypes of Fronts 1. Cold Front: The zone where cold air is replacing warmer air • In U.S., cold fronts usually move from northwest to southeast • Air gets drier after a cold front moves through Examples Fronts: Four Types of Fronts 2. Warm Front: The zone where warm air is replacing colder air • In U.S., warm fronts usually move from southwest to northeast • Air gets more humid after a warm front moves through Examples Fronts: Four Types of Fronts 3. Stationary Front: When either a cold or warm front stops moving • When the front starts moving again it returns to either being a cold or warm front Fronts: Four Types of Fronts 4. Occluded Front: Formed when a cold front overtakes a warm front • This occurrence usually results in storms over an area • In U.S., the colder air usually lies to the west