Weather Patterns

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Weather Patterns
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Air masses

• Air mass Large body of air
• Air masses are about 1000 miles or across and
  about 2 miles thick
  

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Types of air masses

• Polar and arctic air masses Air masses that
  originate at high latitudes, usually towards the
  poles
  
• Tropical air masses Air masses that originate
  low latitudes, mainly around the tropics
  
• Continental air masses Form over land
• Maritime air masses Form over water

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Air masses (cont.)

• Types based on classification
• 1) continental polar (cP)
• 2) continental tropical (cT)
• 3) maritime polar (mP)
• 4) maritime tropical (mT)
• 5) continental arctic (cA)

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Continental polar (cP) air masses

• Continental polar and maritime tropical air
  masses influence weather in North America the
  most
  
• cP air masses originate in northern Canada,
  Alaska, and the Arctic
  
• These air masses are responsible for lake
  effect snows
  
• cP air masses bring heavy snow in most of North
  America

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Maritime tropical (mT) air masses

• mT air masses affecting North America originate
  over the warm waters of the Gulf of Mexico,
  Caribbean Sea, and the Atlantic Ocean
  
• These air masses are warm, moist, and usually
  unstable
  
• mT air is responsible for most of our
  precipitation
  
• In summer, these air masses bring about high
  temperatures and oppressive humidity

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Continental tropical (cT)

• This air mass is definitely prevalent in the
  Southwest and Mexico, especially during the
  summer months

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Fronts

• These are boundaries that separate different air
  masses
  
• Fronts can form between any two contrasting air
  masses
  
• Fronts are relatively narrow, basically 9-120
  mile bands of discontinuity
  
• Ideally, air masses on both sides of a front move
  in the same direction at the same speed

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Types of fronts

• Warm fronts
• Cold fronts
• Stationary fronts
• Occluded fronts

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Warm front

• A front along which a warm air mass overrides a
  retreating mass of cooler air
  
• On a weather map, warm front is shown with a red
  line and semicircles protruding into the cooler
  air
  
• The first sign of the approach of a warm front is
  the appearance of cirrus clouds ahead

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Warm front (cont.)

• As the front nears, lower clouds and then thicker
  stratus and nimbostratus clouds form, bringing
  moderate or light precipitation
  
• As warm front passes, gradual increase in
  temperature occurs

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Cold front

• Front in which dense, cold air occupies territory
  formerly taken by warm air
  
• Cold fronts are about twice as steep as warm
  fronts
  
• Cold fronts advance faster than warm fronts
• As cold front approaches, towering cumulonimbus
  clouds form, which account for most of our
  violent weather

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Cold fronts (cont.)

• In a cold front, intense precipitation over a
  shorter time duration results
  
• As this front passes, a temperature drop and a
  wind shift results

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Stationary front

• Front in which the surface position does not
  move
  
• Neither the cold air mass or the warm air mass
  overtake each other
  
• The front can remain over an area for several
  days
  
• Stationary fronts give weather conditions similar
  to that of a warm front (light to moderate
  precipitation)

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Occluded front

• Front in which an active cold front overtakes a
  warm front
  
• Precipitation occurs along both frontal
  boundaries, combining the narrow band of heavy
  precipitation of a cold front with a wider band
  of precipitation of a warm front

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Middle latitude cyclones

• Usually shown on weather maps depicting an L,
  meaning low pressure system
  
• Large centers of low pressure that travel from
  west to east
  
• Weather conditions usually last from a few days
  up to a week
  
• These systems bring precipitation and severe
  weather

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Thunderstorms

• A storm that generates lightning and thunder
• Produced in cumulonimbus clouds
• Thunderstorms are formed when warm, humid air
  rises in an unstable environment

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Thunderstorms (cont.)

• An estimated 16 million thunderstorms occur every
  year
  
• At any given moment, about 2000 thunderstorms are
  in progress over different parts of the earth
  
• All thunderstorms require warm, moist air

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Thunderstorms (cont.)

• Most thunderstorms take place in the early
  evening, when surface temperatures are high

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Tornadoes

• These are violent windstorms that take on the
  form of a rotating column of air (vortex) that
  extends from a cumulonimbus cloud
  
• Tornadoes form in association with thunderstorms
  that produce high winds, and torrential rainfall
  

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Tornadoes (cont.)

• The most intense tornadoes are the ones which
  form in association with huge thunderstorms
  
• General atmospheric conditions for tornadoes
  usually involve the encounter of a cold air mass
  with a moist warm air mass
  
• Most tornado activity occurs in the central part
  of the U.S.

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Tornadoes (cont.)

• Tornadoes can occur any month of the year,
  April-June is the period of greatest tornado
  activity

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Hurricanes

• A tropical cyclonic storm with winds in excess of
  74 miles per hour
  
• Hurricanes are the largest and most destructive
  storms on earth
  
• Hurricanes can impose damage in the billions of
  dollars and cause great loss of life

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Hurricanes (cont.)

• Hurricanes form in all the tropical oceans,
  except for the South Atlantic and South Pacific
  Oceans
  
• In the western Pacific Ocean, they are called
  typhoons, in the Indian Ocean, they are called
  cyclones

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Profile of a Hurricane

• Hurricanes average 375 miles across
• Hurricanes are low pressure systems
• Lowest barometric pressures ever recorded in the
  Western hemisphere are associated with
  hurricanes
  

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Eye

• Eye Calmest portion of the storm
• In the eye, precipitation ceases and winds
  subside
  
• The eye is the warmest part of the storm
• The eye is characterized by clear, blue skies

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Eyewall

• Most intense portion of the storm
• In the eyewall, the greatest windspeeds and
  heaviest rainfall occur

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Hurricanes (cont.)

• Hurricanes are fueled by the heat liberated when
  huge quantities of water vapor condense
  
• Hurricanes most often in late summer, when ocean
  waters have reached temperatures of 80oF or
  higher
  

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Hurricanes (cont.)

• When a cyclones strongest winds do not exceed 38
  miles per hour, it is called a tropical
  depression
  
• When winds are between 38-74 miles per hour, it
  is called a tropical storm
  
• When a tropical storm is produced, it is then
  assigned a name

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Saffir-Simpson scale

• This scale ranks the relative intensity of
  hurricanes
  
• 75-96 mph Category 1
• 97-111 mph Category 2
• 112-131 mph Category 3
• 132-155 mph Category 4
• 155 mph Category 5

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Hurricane destruction

• Hurricane damage is divided into three
  categories 1) wind damage 2) storm surge and
  
• 3) inland freshwater flooding

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Hurricane damage (cont.)

• Storm surge is responsible for 90 percent of
  hurricane deaths
  
• Storm surge An abnormal rise of the sea along a
  shore as a result of strong winds

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Forecasting tornadoes

• Tornado watch Alerts the public of the
  possibility of tornadoes in a specified area
  
• Tornado warning Issued when a tornado has
  actually been sighted and is indicated by radar
  
• Accuracies of tornado warning by the use of
  Doppler radar