Weather Patterns and Severe Storms

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Weather Patterns and Severe Storms

• Earth Science CH 20

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• Air Masses create weather events such as heat
  waves, cold spells, stormy weather, and bitter
  cold periods.
• Large bodies of air called air masses are
  characterized by similar temperatures and amounts
  of moisture at any given altitude.

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• Air masses can be 1600 km or more across and
  several kilometers thick
• Usually takes several days to move over an area.
• An area experiences days of similar weather.

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• As an air mass moves, it carries its temperature
  and moisture conditions with it.
• Cold dry air from northern Canada warms as it
  heads south but brings the coldest weather of the
  winter to the places in its path.
• As the air mass moves, the characteristics may
  change. It may warm as it moves, it may pick up
  water and cause precipitation.
• Air masses are characterized by the surface over
  which it forms.

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• This is called its source region.
• c continental air masses form over land (dry)
• m maritime air masses form over water (humid)
• these words describe the moisture content

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Using these symbols there are four basic types of
air masses
cP continental polar air is dry and cool cT
continental tropical air is dry and warm mP
maritime polar air is cold and humid mT
maritime tropical air is warm and humid.
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• Much of North America, especially east of the
  Rocky Mountains is affected by cP and mT air
  masses.
• Notice that mT air masses can form over the Gulf,
  the Caribbean, or the Atlantic.
• cT air masses affect very little of the US.
  Their effect is felt in Mexico and extreme
  southwestern states in the summer. Occasionally,
  if they move out of their source zone, they can
  cause drought in the Great Plains or Indian
  summer in the Great Lakes region in the fall.

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• cP are cold and dry and winter and cool and dry
  in summer. They bring clear skies and cooooooold
  weather.
• If a cP crosses one of the Great Lakes, it brings
  heavy snows to Buffalo, Rochester and cities east
  of these large bodies of water. The water is
  much warmer relative to the cold air. This
  causes humid and unstable air making heavy snow
  possible.

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• mT air is responsible for most of the
  precipitation in the eastern two thirds of the
  US. They also bring humid air with high
  temperatures in the summer. (Sound like North
  Carolina?)
• mP air might originate as cP air over Siberia,
  cross the Pacific and affect the weather in the
  North West.
• mP air from the Atlantic affects eastern Canada
  and New England causing storms called noreasters.

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• When two air masses meet, they form a front. A
  front is a boundary that separates two air masses
• There are two types of fronts.

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• Warm fronts form when warm air moves into an area
  formerly covered by cool air. On a weather map
  it is shown by a red line with red semicircles
  that point toward the cooler air.
• They are gradual, have a particular sequence of
  clouds
• They form light to moderate precipitation.

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• Cold fronts form when cold, dense air moves into
  a region occupied by warmer air. This is shown
  as a blue line edged with blue triangles that
  point toward the warmer air mass.
• Cold fronts cause forceful lifting of air. They
  lead to heavy downpours and gusty winds.
• When the cold front passes, the temperature drops
  and the wind shifts.
• Cold fronts are usually followed by clear weather.

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• A stationary front occurs when the surface
  position of the front does not move. This is
  shown on a weather map by blue triangles on one
  side of the front and red semicircles on the
  other.
• Sometimes this produces gentle to moderate
  precipitation.
• Occluded fronts occur when an active cold front
  overtakes a warm front.

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• The main weather producers in the US are middle-
  latitude cyclones.
• They are indicated on a weather map by the letter
  L
• They are large centers of low pressure that
  generally travel from west to east.
• They have a cold front and frequently a warm
  front.

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• They develop in the following way
• Two air masses with different temperatures move
  in opposite directions.
• Over time the front takes on a wave shape.
• Warm air moves toward the pole.
• It takes the place of colder air.
• Cold air moves toward the equator.

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• This results in air moving in a counterclockwise
  direction.
• Cold fronts move faster than warm fronts.
  Eventually the cold front closes in and lifts the
  warm front. As the occlusion begins the storm
  often gets stronger. In the winter this can
  produce heavy snow. In a day or two the warm
  front is displaced and only cold air surrounds
  the cyclone. The energy source is gone.
  Friction slows the airflow and the
  counterclockwise flow stops.

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• Air high in the atmosphere fuels middle latitude
  cyclones. P-570

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• Thunderstorms A storm that generates lightning
  and thunder, gusty winds, heavy rain, and,
  sometimes, hail.
• About 45,000 thunderstorms take place each day.
• At any given time there are about 2000 going on.

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• They have three stages
• Cumulus stage warm air supply moisture to the
  clouds, they grow vertically
• As the cloud grows, the amount and size of the
  precipitation is too great for updrafts to
  support. Heavy precipitation begins.
• Eventually downdrafts dominate, cooling the Earth
  and stopping the updraft of hot, humid air
  However, fresh supplies of warm humid air can
  continue to rise on the edges of the cell.
  Thunderstorms rarely last more than an hour or
  two.

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• Tornadoes. These are more violent storms that
  take the form of a rotating column of air called
  a vortex. They extend downward from a
  cumulonimbus cloud
• A mesocyclone has to form see p 573.
• The updrafts of the thunderstorm causes the
  mesocyclone to become vertical.
• The pressure inside the tornado is very low.
  This causes air near the ground to rush in from
  all directions

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• Hurricanes occur in tropical waters. They have
  the name, typhoons, cyclones and tropical
  cyclones.
• They are the most powerful storms on Earth.
• They form between 5 and 20 degrees north and
  south latitude.

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• It is a heat engine that is fueled by the energy
  when huge quantities of water vapor condense.
• On the sides of the storm, air is funneled upward
  because there is an inward rush of warm, moist
  air. The air then turns upward and rises in a
  ring of cumulonimbus clouds. In the center of
  the storm, air descends rapidly producing hot air
  with no precipitation. This is the eye of the
  storm.

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