Section 17.1 Notes Weather changes as air masses move

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Section 17.1 NotesWeather changes as air masses
move
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Air Masses Are Bodies of Air

• Air Mass A large volume of air in which
  temperature and humidity are nearly the same in
  different locations at the same altitude.
• An air mass forms when the air over a large
  region of Earth sits in one place for many days.
  The air gradually takes on the characteristics of
  the land or water below it.
• Earths surface can affect the air on Earth.

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The Four Types of Air Masses

• Continental air masses form over land. Air
  becomes dry as it loses moisture.
• Maritime air masses form over water. Air becomes
  moist as it gains water vapor.
• Tropical air masses form near the equator. Air
  becomes warm as it gains energy from the warm
  land or water below.
• Polar air masses form far from the equator. Air
  becomes cool as it loses energy from the cold
  land or water below.

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The Four Types of Air Masses
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Movement of an Air Mass

• Air masses can travel away from the regions where
  they form. They move with the global pattern of
  winds.
• When an air mass moves to a new region, it
  carries along its characteristic moisture and
  temperature.

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Cold Front

• Front A boundary between air masses.
• A cold front forms when a cold air mass pushes a
  warm air mass and forces the warm air mass to
  rise.
• Cold fronts often produce cumulonimbus clouds,
  which can cause heavy storms.

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Warm Front

• A warm front forms when a warm air mass pushes a
  cold air mass and the warm air rises slowly over
  the cold air.
• As a warm front approaches, you may first see
  high cirrus clouds, then stratus clouds, then
  lower stratus clouds.
• Warm fronts often brings many hours of steady
  rain or snow.

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Stationary Front

• A stationary front occurs when 2 air masses push
  against each other without moving.
• A stationary front becomes a warm or cold front
  when one air mass advances.

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Weather Fronts
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High Pressure Systems

• Formed when air moves all the way around a
  high-pressure center.
• Air sinks and spreads out towards locations with
  low pressure.
• Generally brings clear skies and calm air or
  gentle breezes.

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Low Pressure Systems

• A large weather system that surrounds a center of
  low pressure.
• Air circles and moves upward.
• The rising air produces stormy weather.

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Section 17.2 NotesLow-pressure systems can
become storms
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Hurricanes form over warm ocean water

• Tropical storm a low-pressure system that
  starts near the equator and has winds that blow
  at 65 km per hour.
• Hurricane a tropical low-pressure system with
  winds blowing at speeds of 120 km per hour.

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Formation of Hurricanes

• Hurricanes most often strike between the months
  of August and October.
• Ocean water has to be 80oF or higher for a
  hurricane to form.
• Tropical storms and hurricanes normally move west
  due to the trade winds.
• Hurricanes die out when they move over land.

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Formation of Hurricanes

• The center (eye) of a hurricane is calm.
• The eye wall (surrounds the eye) has heavy rain
  and tremendous winds.

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Effects of Hurricanes

• Hurricanes can lift cars, uproot trees, and tear
  the roofs off buildings due to strong winds.
• Heavy rain can also cause severe flooding.
• Storm surge A rapid rise in water level in a
  coastal area that occurs when a hurricane pushes
  a high mass of ocean water, often leading to
  flooding and widespread destruction.

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Winter storms produce snow and ice

• Winter storms are formed when 2 air masses
  collide (a cold, dry, dense air mass and a moist
  air mass).
• Blizzards Blinding snowstorms with winds at
  least 56 km per hour and low temperatures.
• Lake-Effect Snowstorms Cold air gains moisture
  and warmth as it passes over a lake and then
  moves over cold land where it cools again and
  snows.

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Winter storms produce snow and ice

• Ice Storms When rain falls onto freezing-cold
  ground and covers everything with heavy, smooth
  ice.

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Section 17.3 NotesVertical air motion can cause
severe storms
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Thunderstorms form from rising moist air

• Thunderstorm A storm with lightning and
  thunder.
• Can form at a cold front or within an air mass.

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Formation of Thunderstorms Step 1

• Rising humid air forms a cumulus cloud. The
  water vapor releases energy when it condenses
  into cloud droplets. This energy increases the
  air motion. The cloud continues building up into
  the tall cumulonimbus cloud of a thunderstorm.

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Formation of Thunderstorms Step 2

• Ice particles form in the low temperatures near
  the top of the cloud. As the ice particles grow
  large, they begin to fall and pull cold air down
  with them. This strong downdraft brings heavy
  rain or hail the most severe stage of a
  thunderstorm.

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Formation of Thunderstorms Step 3

• The downdraft can spread out and block more warm
  air from moving upward into the cloud. The storm
  slows down and ends.

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Effects of Thunderstorms

• Flash floods heavy rainfall in the same area
  that can cause rivers to overflow their banks.
• Winds winds as high as 170 mi/hr that can knock
  down entire forests.
• Hail can wipe out entire fields, damage roofs,
  and kill livestock.
• Lightning damage power lines and equipment,
  cause forest fires, and kill people.

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Tornadoes form in severe thunderstorms

• Tornado A violently rotating column of air
  stretching from a cloud to the ground.
• More tornadoes occur in North America than
  anywhere else in the world.
• Occur when warm air masses meet cold, dense air
  and form thunderstorms normally in the central
  plains of the United States.

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Effects of Tornadoes

• Powerful winds can pick up or destroy everything
  in the path of a tornado.
• Most tornadoes are small and last only a few
  minutes.
• Larger tornadoes are less common but have
  stronger winds and last longer.
• The path of a tornado is unpredictable.