Chapter Twenty

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Chapter Twenty

• Weather

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Topic OneAir Masses and Weather

• Scientists classify an air mass based on whether
  it originates in an arctic, in a polar, or in a
  tropical region and whether it forms over land
  (continental) or sea (maritime.) An air mass is
  a large body of air with uniform temperature and
  humidity. Polar air masses and tropical air
  masses influence the weather of North America.

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Topic TwoFronts and Lows

• A front is the band of air between opposing air
  masses. Scientists classify a front based on the
  temperature of the advancing air mass. Cold and
  warm fronts are associated with characteristic
  weather conditions. Fronts are usually connected
  to mid-latitude, low-pressure systems. Upper
  level air flow influences the convergence or
  divergence of air into and out of pressure
  systems.

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Topic FourHurricanes and Winter Storms

• Hurricanes are large rotating storms originating
  over tropical oceans. They are classified based
  on wind speed. Winter storms are
  middle-latitude, low-pressure systems that occur
  over land in the winter.

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Topic ThreeThunderstorms and Tornadoes

• Thunderstorms form in warm, moist, unstable air.
  They produce lightening, a discharge of
  electricity. Tornadoes can develop in
  thunderstorms containing rotating updrafts.

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Topic FiveForecasting Weather

• Weather forecasters must gather huge amounts of
  data in order to make their predictions. They
  rely on sensing instruments and computer models
  to provide the information they need.
  Meteorologists make both daily and long-term
  forecasts of the weather.

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Key Terms

• air mass blizzard
• cold front front
• hurricane lightening
• occluded front Saffir-Simpson scale
• squall line supercell
• tornado thunderstorm
• station model stationary front
• storm surge warm front

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Topic OneAir Masses and Weather

• Differences in air pressure at different
  locations on earth create wind patterns. The
  equator receives more solar energy than the
  poles, heating the air, causing it to rise.
  Cold, polar air sinks. As air moves from high to
  low pressure, a general, worldwide movement of
  surface air from the poles toward the equator
  occurs. Temperature and pressure differences on
  the surface alter this. Three convection cells
  are created in the Northern hemisphere and three
  in the southern hemisphere. The Coriolis Effect
  also affects the movement of winds.

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Air Mass

• A VERY large mass of air that sits over an area
  for days or weeks and picks up the temperature
  and humidity characteristics of that area.
• They can be thousands of kilometers in diameter.

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• Continental Land
• Maritime Water
• Polar Cold
• Tropical Warm

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Maritime Polar (mP)

• An air mass that forms over a cold, wet area.

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Maritime Tropical (mT)

• An air mass that forms over a warm, wet area.

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Continental Polar (cP)

• An air mass that forms over a cold, dry area.

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Continental Tropical (cT)

• An air mass that forms over a warm, dry area.

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• Air masses strongly affect the weather on North
  America. As an air mass moves away from the area
  over which it formed, it can change. Cold, dry
  air can become warmer and more moist as it moves
  from land over the warmer ocean, for example.

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• Three polar air masses influence the weather
  over North America. They are called
• 1. continental polar Canadian
• 2. maritime polar Pacific
• 3. maritime polar Atlantic

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Critical Thinking

• Where do each of the air
• masses form?

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• Suppose snow is falling on the Pacific coast
  area. What type of air mass is probably
  responsible for this weather?

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• cP air masses move southeast from Canada into
  the United States. It brings very cold, dry air.
  mP Pacific air masses are moist and cool, but
  not cold. They bring cool, foggy weather to the
  Pacific northwest and drop their moisture as the
  head over the mountain ranges of the western U.S.
  mP Atlantic air masses move east toward Europe,
  passing over New England and maritime Canada,
  bringing cool, cloudy, wet weather.

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Homework

• Read Pages 436 to 438
• Complete Worksheet Air Masses and Weather

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Topic TwoFronts and Lows

• When two unlike air masses meet, density
  differences keep the masses separate. Cool air
  is dense and does not mix with warm, less dense
  air. In order for a front to form, one air mass
  must collide with another.

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Front

• A place where two different air masses meet.

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Cold Front
The boundary between an advancing cold air mass
and the warm air mass it is displacing.
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• The moving cold air pushes up the warm
• air. If the air is moist, large cumulus and
• cumulonimbus clouds form. Short-lived,
• sometimes violent storms result. A squall
• line, a line of heavy thunderstorms,
• may occur just ahead of a fast-moving
• cold front. Slow-moving cold fronts produce
• little rain and less cloudiness.

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Warm Front

• The boundary between an advancing warm air mass
  and the cold air mass it is displacing.

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• The slope of a warm front is gradual. This
  allows clouds to appear a long time before the
  base of the front. High cirrus clouds appear
  first, followed by cirrostratus, altostratus, low
  stratus, and finally nimbostratus at the base of
  the front. Precipitation is produced over a
  large area.

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Occluded Front

• The front that is formed when a cold front
  overtakes a warm front and displaces it upward in
  an area of low pressure.

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• The advancing cold front comes in contact with
  the cool air underneath the lifted warm air,
  cutting off the warm front from the ground below,
  holding it high in the atmosphere.

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• When two air masses meet and neither is
  displaced, a stationary front results. The two
  air masses move parallel to the front between
  them. The weather around a stationary front is
  similar to that produced by a warm front.

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Draw the symbol for a cold front.
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Draw the symbol for a warm front.
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Draw the symbol for an stationary front.
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Draw the symbol for a occluded front.
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• Over each of earths polar regions, a dome of
  cold air exists. The boundary where this frigid
  air mass meets the warmer air of the middle
  latitudes, a polar front exists. This front
  circles the earth between 40º to 60º latitude in
  each hemisphere. Waves often develop along polar
  fronts.

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• A wave is a bend in a stationary front or cold
  front. The jet stream helps develop these waves.
  Waves along the boundary of a polar front or a
  cold front and produce low-pressure storm centers
  called wave cyclones. These large storms can be
  up to 2,500 km in diameter. Their winds blow in
  circular paths spiraling upward around the
  low-pressure center. They strongly influence the
  weather in the middle latitudes.

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Wave cyclones occur along a cold or stationary
front.

• 1. Surface pressure at a polar front drops and a
  wave forms on the front. The pressure is lowest
  at the top.
• 2. Winds blow CCW around the low. Warm air on
  the east moves north while cold air on the south
  moves south.
• 3. The cold front catches up to the warm front,
  resulting in an occluded front. The storm is at
  its highest intensity.

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• An anticyclone is the opposite of a cyclone,
  which sinks and flows out from the center.
  Because of the Coriolis Effect, the circulation
  around an anticyclone is clockwise in the
  Northern Hemisphere. Wave cyclones bring cloudy,
  stormy weather, while anticyclones bring dry
  weather. If an anticyclone stagnates over an
  area, it can cause air-pollution problems.

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• Tropical storms differ from mid-latitude storms
  in many ways. They are concentrated over a small
  area, lack cold and warm fronts, and are usually
  more violent and destructive than most wave
  cyclones.

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Homework

• Read Pages 439 to 444.
• Complete Worksheet Fronts

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Topic ThreeThunderstorms and Tornadoes

• A thunderstorm develops in three stages
• 1. Cumulus stage is when warm moist air rises
  and condenses to form cumulus clouds.
• 2. Mature stage is when cumulonimbus clouds
  release torrential rains and there is cooling.
• 3. Dissipating stage is when down drafts and
  rain stops.

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• During a thunderstorm, clouds discharge
• electricity in the form of lightening.
• The released electricity heats the air,
• causing it to expand rapidly. The expansion
• and release of the air causes thunder. For
• lightening to occur, the clouds must have
• areas with different electrical charges.
• The upper part will carry a positive charge,
• while the lower part will carry a negative
• one. About 10 to 20 of the lightening
• strikes the ground. The remainder goes
• from cloud to cloud, or cloud to space.

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Squall Line

• A line of thunderstorms that occur ahead of a
  front. There are often strong winds before the
  rain.
• Strong winds often precede squall lines. This
  happens because the rain falling within a
  thunderstorm causes the air to cool, becoming
  more dense, and sinking. This is called a
  downdraft.

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Super Cell

• A very large, single thunderstorm with very
  strong updrafts.
• Strong winds often precede squall lines. This
  happens because the rain falling within a
  thunderstorm causes the air to cool, becoming
  more dense, and sinking. This is called a
  downdraft.

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Lightning

• A discharge of electricity from a thundercloud
  to the ground, to another cloud, or to another
  spot within the cloud itself.

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Tornado

• A violent, rotating column of air that extends
  down from dark clouds and moves overland in a
  narrow, destructive path.
• They may reach speeds of 400 km/hr.

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Critical Thinking

• Suppose a hurricane is passing over a Caribbean
  island. Suddenly, the rain and wind stops and
  the air becomes calm and clear. Is it safe to go
  outside? Explain your answer.

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Homework

• Read Pages 445 to 449.

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Topic FourHurricanes and Winter Storms
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Hurricane

• A large, rotating storm of tropical origin with
  sustained winds of at least 119 km/hr.

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• They are seldom more than 700 km in diameter,
  and are most powerful of all storms. They occur
  in both the Atlantic and Pacific. More occur in
  the Pacific, an average of 20 per year. In the
  Pacific, they are called typhoons.

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• They occur when
• 1. Warm, moist air evaporates off the surface
  of the ocean and rises rapidly
• 2. The moisture condenses, releasing latent
  energy in the form of heat
• 3. The heat increases the force of the rising
  air
• 4. The patterns continues, sustaining the
  process

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• An average hurricane has the same amount of
  energy as all the electricity used in the U.S. in
  6 months.

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Storm Surge

• A rapid rise in water level along the coast as a
  hurricane or other tropical storm approaches.

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Saffir-Simpson Hurricane Scale

• The 1 to 5 scale used to rate a hurricanes
  intensity and estimate potential property damage
  and flooding.

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• Winter storms are mid-latitude low pressure
  systems that can bring several types of weather,
  including heavy snow, ice, and rain. When
  conditions become severe, a blizzard may occur.
  A blizzard is characterized by high winds, low
  temperatures and falling or blowing snow.

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• To be considered a blizzard, three conditions
  must be met.
• 1. Winds exceed 56 km/hr.
• 2. Temperatures are below - 7C.
• 3. Falling and/or blowing snow reduce
  visibility.

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• On the east coast, noreasters occur over the
  north Atlantic where extremely heavy snow
  consisting of a maritime polar air mass is blown
  from the ocean to the land.

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Homework

• Read Pages 450 to 453.
• Complete Worksheet Hurricanes and Tornadoes

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Topic FiveForecasting Weather

• Weather can be measured at both the surface of
  the earth and above the surface.

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• As discussed in the last chapters, at the
  surface, psychrometers, hygrometers, and
  barometers are used to measure humidity. Rain
  gauges measure the amount of precipitation.

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• Technology has allowed satellites to provide
  weather data for every place on Earth.
  Scientists use both visual and infrared images
  from satellites. Infrared uses temperature to
  provide a color image based on the temperature
  differences.

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• Above the earths surface, an instrument package
  called a radiosonde measures pressure,
  temperature and relative humidity. A
  helium-filled balloon carries the radiosonde
  aloft. Measurements are sent back to the surface
  by radio waves. When an extremely high altitude
  is reached, the balloon bursts, and the
  electronic package is parachuted back to earth.

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RADAR

• A method of detecting distant objects and
  recording their features and properties by
  looking at the electromagnetic waves that are
  reflected from their surface.

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• Supercomputers and satellites are also used.
  Complex mathematical equations help explain the
  behavior of the atmosphere.

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• Predicting the weather has challenged human
  society for thousands of years. Forecasting
  began when basic instruments were invented. In
  1844, when the telegraph was invented, weather
  information could be sent from place to place
  quickly. In 1870, the Army Signal Corps formed
  the first weather forecasting agency. In 1970,
  it was renamed the National Weather Service. In
  1873, the World Meteorological Organization was
  formed. It is now part of the United Nations.

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• Coded weather information is put on maps
  prepared by centers around the world. Cluster of
  symbols are plotted for each station.

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Station Model

• A picture representation for all of the data
  that make up a weather report for a particular
  location at a particular time.

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• Meteorologists make two types of forecasts,
  daily, and long-term. The most recent map is
  compared to one made 24 hours ago. Satellite
  images and radar reports are computerized and
  along with a meteorologists personal experience,
  are used to help make maps. Accurate forecasts
  can be made for 3 to 5 days. Extended forecasts
  for 6 to 10 days can be made by computer. Long
  term forecasts can show general patterns, but are
  not that accurate. Plotting the path of a
  hurricane is an example. We can tell about where
  it will go over a period of hours, but can not
  know days in advance.

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• While scientists have had success predicting the
  weather, they have had very little controlling
  it. Cloud seeding, to break up potential
  hailstorms has been tried. Seeding of hurricanes
  has also been tried. Attempts to control
  lightening have also been made.

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Critical Thinking

• Write todays weather in the spaces provided
  below. Use the data from the weather station to
  write your report.

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Make the station model for your report.

• -2 996
• 1/4
• 30 28/

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• Surface weather maps are used to present a
  picture representation of current weather
  conditions. In locating fronts, the following
  guidelines are used
• 1. Wind direction changes behind fronts
• 2. Temperature changes sharply across fronts
• 3. Dew point changes sharply across fronts

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Homework

• Read Pages 455 to 459.
• Complete Worksheet Forecasting the Weather