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Severe Weather
Air masses are classified on the
basis of their source region
Air masses and weather
 Continental

polar (cP)
From northern Canada and interior of Alaska
• Winter – brings cold, dry air
• Summer – brings cool relief

Responsible for lake-effect snows
• cP air mass crosses the Great Lakes
• Air picks up moisture from the lakes
• Snow occurs on the leeward shores of the lakes
Air masses and weather
 Maritime


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tropical (mT)
From the Gulf of Mexico and the Atlantic
Ocean
Warm, moist, unstable air
Brings precipitation to the eastern United
States
Air masses and weather
 Continental



tropical (cT)
Southwest and Mexico
Hot, dry
Seldom important outside the source region
Air masses and weather
 Maritime


polar (mP)
Brings precipitation to the western mountains
Occasional influence in the northeastern
United States causes the "Northeaster" in
New England with its cold temperatures and
snow
Fronts
 Types

of fronts
Warm front
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Warm air replaces cooler air
Shown on a map by a line with semicircles
Small slope (1:200)
Clouds become lower as the front nears
Slow rate of advance
Light-to-moderate precipitation
Warm Front / High
pressure system
Rain!
Cold Front / Low
pressure system
Fronts
 Types

of fronts
Cold front
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Cold air replaces warm air
Shown on a map by a line with triangles
Twice as steep (1:100) as warm fronts
Advances faster than a warm front
Associated weather is more violent than a warm
front
• Weather behind the front is dominated by
• Cold air mass
• Subsiding air
• Clearing conditions
Cold Front / Low
pressure system
Rain/Snow
Thunderstorms
Warm Front / High
pressure system
Fronts
 Types

of fronts
Stationary front
• Flow of air on both sides of the front is almost
parallel to the line of the front
• Surface position of the front does not move

Occluded front
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Active cold front overtakes a warm front
Cold air wedges the warm air upward
Weather is often complex
Precipitation is associated with warm air being
forced aloft
Middle-latitude cyclone
 Primary
weather
producer in the
middle-latitudes
Middle-latitude cyclone
 Idealized

weather
Middle-latitude cyclones move eastward
across the United States
• First signs of their approach are in the western sky
• Require two to four days to pass over a region

Largest weather contrasts occur in the spring
Cloud
patterns
typically
associated
with a
mature
middlelatitude
cyclone
Stages in the development
of a thunderstorm
Severe weather types
 Tornadoes



A rapidly spinning column of air descending
from a large thunderstorm
Have the highest wind speeds of any other
weather phenomenon.
In the U.S., tornadoes move northeast at
speeds up to 100 km/hr (62 mph)
• Rotating wind speeds sometimes in excess of 500
km/hr (310 mph)
Severe weather types
 Tornadoes

How a Tornado
Forms
• Moist air from Gulf
of Mexico
• Fast moving cold,
dry air mass from
Canada
• Jet stream moving
east at 150 mph
• Sets up shearing
conditions
Severe weather types
 Tornadoes

How a Tornado
Forms
• Warm moist Gulf air
releases latent
heat, creates strong
updraft
• Updraft sheared by
polar air, then
twisted in a different
direction by jet
stream
Severe weather types
 Tornadoes


Why do some thunderstorms spawn
tornadoes while others do not?
Super Cell Thunderstorms
Severe weather types
 Tornadoes

Scales
• The Fujita-Pearson Scale measures wind speed


The size of a tornado is not necessarily an indication
of its intensity
Large tornadoes can be weak, and small tornadoes can
be violent
Severe weather types
 Hurricanes

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Only natural disaster that is given a human
name
Actually large tropical cyclones
Convert heat in the ocean into winds
Exports excess heat from the tropics to the
midlatitudes
Severe weather types
 Hurricanes

How a Hurricane Works
• Tropical disturbance

Low pressure zone develops and draws in clusters of
thunderstorms and winds
Severe weather types
 Hurricanes

How a Hurricane Works
• Tropical disturbance
• Tropical depression

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Surface winds strengthen, move about the center of the
storm
Central core funnels warm moist air up towards
stratosphere
Air cools, vapor condenses, latent heat released
Fuels more updrafts, cycle repeats, storm grows
Severe weather types
 Hurricanes

How a Hurricane Works
• Tropical disturbance
• Tropical depression
• Tropical Storm

Storm has sustained surface wind speeds of +39 mph
Severe weather types
 Hurricanes

How a Hurricane Works
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Tropical disturbance
Tropical depression
Tropical Storm
Hurricane
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Surface winds consistently over 74 mph
Severe weather types
 Hurricanes

How a Hurricane Works
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Tropical disturbance
Tropical depression
Tropical Storm
Hurricane
The Eye
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As wind speed increases, winds are spiraled upwards
prior to reaching the center
A distinctive clear “eye” is formed
Strongest winds are located on the walls of the eye
Hurricane Origins
in the tropics ~ 5° and 20 ° latitude
 Cannot form at the equator
(Coriolis effect = 0)
 Form
Hurricanes
 Hurricane

Damages
Storm Surges
• Large mound of water builds up beneath the eye
• Reaches land as a surge of water
Hurricanes
 Hurricane
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Damages
Storm Surges
• Wind speed varies
depending upon which
side of the hurricane
you’re on
• Amount of damage on the
coastline will vary
accordingly
Hurricanes
 Hurricane

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Damages
Heavy Rains
Mudflows and Debris Avalanches
Flooding
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