Weather

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Weather

• Chapter 20

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20.1 Air Masses and Weather

• Meteorology- the study of the processes that
  govern the earths atmosphere
• Air mass- a large body of air that has similar
  conditions throughout
• The air mass takes the conditions of the area in
  which it forms

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20.1 Air Masses and Weather

• Air Mass types
• Continental arctic
• Form in the direction of the north pole
• Often move down into Wisconsin and create near
  record cold temperatures
• Typically very dry
• Continental polar
• Form in the Canada/Alaska area
• Bring cold weather
• Typically very dry
• Often cause lake-effect snow in Michigan and
  Wisconsin as the cold air blows across the warmer
  Lake Michigan

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20.1 Air Masses and Weather

• Maritime Polar
• Originate over the northern oceans
• Usually contain a lot of moisture with air that
  is close to its dew point
• Causes fogs and rains to fall as it moves over
  the Pacific Northwest
• Maritime Tropical
• Originate over warm oceans at the equator
• Warm and humid air
• Often causes muggy weather with a great chance
  for thunderstorms
• Continental Tropic
• Originate over dry land in lower latitudes
• Typically extremely dry and hot
• Record high temperatures are possible when this
  air mass moves in

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20.2 Fronts and Lows

• Front the boundary that separates opposing air
  masses
• Most common at mid-latitudes where cold air from
  the north and warm air from the south constantly
  meet

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20.2 Fronts and Lows

• Front types
• Cold front
• When a cold air mass slides in under a warm air
  mass
• Wedge shaped
• Often result in a line of storms being produced
  as the change in weather can be dramatic
• Symbol-
• Warm front
• When a warm air mass slides in over a cold air
  mass
• Wedge shaped also but the wedge points in the
  opposite direction
• Often result in cloudy weather with light rain as
  the change in weather is slow
• Symbol-

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20.2 Fronts and Lows

• Occluded front When a cold front (they move
  twice as fast) catches up to a warm front which
  results in rain
• Symbol-
• Stationary front- When the two air masses are
  locked in one spot for a period of time
• Results in buckets of rain and possible flooding
• Symbol-

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20.2 Fronts and Lows

• Mid-latitude low
• Occur when a Low pressure area is the boundary
  between a warm air mass and a cold air mass
• The two air masses actually push counterclockwise
  around the low as the warm air attempts to go
  north and the cold air attempts to move south
• This gives us the biggest storms in the summer
  and the winter.
• Our storm this spring that dropped 17 inches was
  from a mid-latitude low
• Upper air flow
• The jet stream feeds this mid latitude low with
  both warm/humid air and cold/dry air to create a
  trough (boundary) of storm systems that can
  stretch across our entire country.

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20.3 Thunderstorms and Tornadoes

• Thunderstorms storms with lightning, thunder,
  rain, and sometimes hail
• Life cycle of a thunderstorm
• Cumulus stage Warm, humid air rises and cumulus
  clouds form
• Mature stage- Cumulus clouds have evolved into
  towering cumulonimbus clouds and the
  precipitation becomes too heavy for the rising
  air (updraft) to hold and rain falls
• Dissipating stage- The falling rain creates a
  downdraft which stops the moist air from rising
  and the cloud begins to evaporate

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20.3 Thunderstorms and Tornadoes

• Squall lines- lines of thunderstorms that develop
  along frontal boundaries
• Supercells- an area within a squall line that has
  particularly strong updrafts where the strongest
  winds, heaviest rain, biggest hail, and most
  destructive tornadoes develop
• How does lightning form?

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20.3 Thunderstorms and Tornadoes

• Tornadoes
• Form from a rotating updraft of air with
  different air at a higher level
• Wall cloud- low level wall of clouds from which
  many tornadoes come
• Often appear on the southwest edges of storms
• Tornado alley area that extends from Texas to
  North Dakota where the most tornadoes occur
• Can have just one vortex or many
• Are often hidden by rain

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20.3 Thunderstorms and Tornadoes

• Categorized on the Fujita scale (F0 F5)
• What do you do in a tornado?
• Can tornadoes be predicted?
• What famous Wisconsin tornadoes are you aware of?
• Stoughton
• Oakfield
• West Bend
• ????????

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20.3 Thunderstorms and Tornadoes

• Watches and Warnings
• Watch the conditions are right for a storm to
  develop, watches are over broad areas
• Warning- a severe storm has developed and is
  headed for you
• Jetstream

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20.4 Hurricanes and Winter Storms

• Hurricanes a large rotating storm of tropical
  origin with sustained winds of 119 km/h
• Formation
• Need a supply of warm moist air for a long period
  of time
• A disturbance occurs which causes the rising
  warm moist air to begin to circulate
• The flow is counterclockwise in the Northern
  Hemisphere
• A tropical depression is when winds top out below
  119 km/h
• Only a few depressions become hurricanes

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20.4 Hurricanes and Winter Storms

• Effects
• Wind damage
• Inland flooding
• Storm surge
• A wall of water that accompanies a hurricane
• If it occurs at the same time as high tide it
  results in great damage
• Watches and warnings
• Watch hurricane is 24-36 hours away
• Warning hurricane is 24 or less hours away
• Categories
• Range from 1-5 with 5 being the worst
• Hurricanes Katrina and Rita were both in the 5
  range at times in their life cycle

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20.4 Hurricanes and Winter Storms

• Winter Storms
• Usually result from mid-latitude lows
• Blizzard
• Winds in excess of 40 mph
• Temperature in the teens
• Greatly reduced visibility from blowing or
  falling snow

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20.5 Forecasting Weather

• Gathering Data
• Satellite images
• Visible images
• Infrared images
• See examples
• Radiosondes
• Surface observations
• Report hourly and often are tied to airports

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20.5 Forecasting Weather

• Making weather maps
• Station model (see p. 457 and p. 458)
• Forecasting
• Use various mathematical formulas to predict
  weather
• They are created 4 times per day by the national
  weather service
• The reliability is greatly reduced beyond a 2 day
  forecast