Chapter 3 Section 1

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Chapter 3 Section 1

• Air Masses and Fronts

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Four Major Types of Air Massesin North America

• Maritime Tropical
• Maritime Polar
• Continental Tropical
• Continental Polar

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Maritime Tropical

• Warm, humid air masses form over oceans near the
  tropics
• Influence weather in the central eastern United
  States when formed over the Gulf of Mexico the
  Atlantic Ocean
• Influence weather on the West Coast when formed
  over the Pacific Ocean
• Summer hot, humid weather
• Winter humid air can bring heavy rain or snow

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Maritime Polar

• Cool, humid air masses form over the icy cold
  North Pacific North Atlantic oceans
• Affect the West Coast more than the East Coast
• In summer, often bring fog, rain, cool
  temperatures to the West Coast

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Continental Tropical

• Hot, dry air masses form only in summer over dry
  areas of the Southwest northern Mexico
• Covers a smaller area than other air masses
• Move northeast, brings hot, dry weather to the
  southern Great Plains

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Continental Polar

• Large masses that form over central northern
  Canada Alaska
• Brings cool or cold air
• In winter, bring clear, cold, dry air to much of
  North America
• Can bring bitterly cold weather with very low
  humidity.
• In summer, storms may occur when air masses move
  south meet maritime tropical air masses moving
  north

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Four Types of Fronts

• Cold Fronts
• Warm Fronts
• Stationary Fronts
• Occluded Fronts

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Cold Fronts

• When a rapidly moving cold air mass runs into a
  slowly moving warm air mass, the denser cold air
  slides under the lighter warm air
• The warm air is then pushed upward resulting in a
  cold front
• Cold fronts move quickly can cause quick
  changes in the weather, including violent
  thunderstorms
• Once a cold front moves through an area, cool,
  dry air moves in and often brings clear skies
  cooler temperatures

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Warm Fronts

• A moving warm air mass collides with a slowly
  moving cold air mass
• As in a cold front, the warm air rises above the
  cold air
• If the warm air is humid, showers light rain
  fall along the front when the two air masses meet
• If the warm air is dry, scattered clouds form
• Warm fronts move much slower than cold fronts,
  therefore, the weather may be rainy or foggy for
  several days
• After a warm front passes through an area, warm
  humid weather will follow
• In winter, warm fronts bring snow

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Stationary Fronts

• Sometimes cold warm air masses meet, but
  neither has enough force to move the other the
  result a stationary front
• When the two air masses meet, water vapor in the
  warm air condenses into rain, snow, fog, or
  clouds
• If the front is stalled, the weather may be days
  of clouds precipitation

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Occluded Fronts

• The most complex of the four fronts
• At an occluded front, a warm air mass is caught
  between two cooler air masses
• The two cooler air masses meet may mix
• Temperature near the ground becomes cooler
• The warm air mass is cut off (occluded) from the
  ground
• As the warm air cools its water vapor
  condenses, the weather may turn cloudy rainy or
  snowy

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Cyclones

• An area of relatively low air pressure is called
  a cyclone
• Low air pressure is represented on a weather map
  as a L
• Cyclone spin counterclockwise in the Northern
  Hemisphere
• Cyclones play a large part in the weather of the
  United States
• As air rises in a cyclone, the air cools, forming
  clouds precipitation
• Cyclones decreasing air pressure are associated
  with storms precipitation

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Anticyclones

• Opposite of cyclones in most ways
• Anticyclones are high-pressure centers of dry air
• High pressure areas are represented on a weather
  map as a H
• Anticyclones spin clockwise in the Northern
  Hemisphere
• Cool air moves downward from higher in the
  troposphere
• As the cool air falls, it warms up, so its
  relative humidity drops
• Descending air in an anticyclone causes dry,
  clear weather