Chapter 20
Weather Patterns and Sever
Storms
Earth Science
Air Masses 20.1
• Immense body of air
that has similar temp
and humidity
• Can be 1500 km or
more
• Air mass carries temp
and humidity from
region in which it
formed but
characteristics change
to take on some climate
of area
Classifying Air Masses 20.1
• Source region – area over which air mass gets
properties
– 7 in US
• POLAR (P) = from high latitudes
– cold
• TROPICAL (T) = from low latitudes
– warm
• Maritime (m) = form over water
– Likely to be humid
• Continental (c) = form over land
– Likely to be dry
Weather in North America 20.1
• Continental Polar (cP)
– Winter: cold and dry; Summer: cool and dry
– Can cause lake-effect snows
• Air gets heat and moisture from water = unstable air mass that
is humid = lots of snow
Weather in North America 20.1
• Maritime Tropical (mT)
– Warm, loaded with moisture = usual
unstable
– Source of most of precipitation in
eastern 2/3 US
• Continental Tropical (cT)
– Least influence on US
– Hot, dry
– Rarely affect weather outside their
source region
Fronts 20.2
• Front – boundary that separates two air masses
• Associated with precipitation
• Types of Fronts (4 Types)
– 1) Warm Front  moves into area formerly covered by
cooler air
• Weather map: red line with semicircles pointing towards cooler air
• Cirrus clouds indicate incoming of warm front
• Usually produces light to moderate precipitation
– 2)Cold Front  forms when cold, dense air moves into
region occupied by warmer air
• Weather map: blue lined edged with blue triangles that point toward
the warmer air mass
• Move more rapidly and have steeper slope
• Heavy downpours and gusty winds
• Weather clears after cold front passes
Fronts 20.2
• Types of Fronts Continued:
– 3) Stationary Fronts  Almost parallel to line of front
• Weather map – blue triangles on one side and red semicircle
on the other
– 4) Occluded Fronts  when active cold front overtake
warm front
• Wedges warm air upward
Middle-Latitude Cyclones 20.2
• Large centers of low pressure and generally travel
west to east
• Cause stormy weather
• Rotate counter clockwise in toward the center
• http://www.youtube.com/watch?v=xYyidKV36xI
– 4 minutes
Severe Storms 20.3
• Thunderstorms – generates lightning and thunder
– Cumulonimbus cloud
– Form when warm, humid air rises in an unstable
environment
• 3 stages
1.
2.
3.
Cumulus Stage – warm moist air is supplied to cloud
Mature stage – Heavy precipitation
Dissipating Stage – cloud begins to evaporate
– Life span: single cumulonimbus cell is about an hour or
two
Tornadoes 20.3
• Rotation column of air called vortex
– Vortex extend downward from cumulonimbus cloud
• 770 reported each year
• Normally from April to June
• A mesocyclone must form for tornado to develop
Tornadoes Continued… 20.3
• Pressure inside tornado is about 10% lower
– Low pressure = air near ground rushes into in all
direction
– Winds can approach 480km/hr
• Intensity measure by the Fujita Tornado Intensity
Scale
Tornado Clips
• http://www.youtube.com/watch?v=GMabtIT6Is
– 1 minute
• http://www.youtube.com/watch?v=75PniEZx
GwE&feature=fvw
– 3 minutes
• http://www.youtube.com/watch?v=KzUTxZkU
5pc&feature=related
– 4 minutes
Hurricanes
•
•
•
•
Whirling tropical cyclones
Winds of at least 119 km/hr
Aka: typhoons, cyclones, tropical cyclones
Most powerful storms on earth
– can generate 15 meter waves
• Most form between 5 – 20 degrees N or S latitude
– North Pacific has around 20 per year
– Most often in late summer when water temps can give
necessary heat and moisture to air
Hurricanes Continued… 20.3
• Eye Wall – doughnut-shaped wall the surrounds the
center of the storm
– Heaviest rainfall and wind speeds here
• Eye – very center
– No precipitation and wind subsides
– Warmest part of storm
Hurricanes Continued… 20.3
• Hurricane Intensity-described by Saffir-Simpson
Scale
– Storm Surge – dome of water about 65 – 80 km wide
that sweeps across the coast
Hurricane Clips
• http://www.youtube.com/watch?v=OEPZOC6YH
Uc
– 3:30 minutes
• http://www.youtube.com/watch?v=HJydFJORWf
4&feature=related
– 5:30