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Cyclones and Thunderstorms
Outline
• Mid-latitude cyclones
– Definitions, scales
– Examples
• Thunderstorms
– How to form
– Associated dangers
Mid-latitude Cyclones
• Key - rotating air body
• At mid-latitudes, these are not hurricanes
• Occur over various scales
– Blizzards to tornados
How do you get a rotating body?
• Helped by bends in polar jet stream
– Create troughs, ridges
Jet Stream Ridges and Troughs
• Troughs
– Low pressure, form cyclones
– Winds flow towards core, rising, cools, rainy weather
• Ridges
– High pressure, form anti-cyclones
– Winds flow down and out over surface, warm dry,
windy conditions common
Scales of Mid-Latitude Cyclones
• Largest: linked to jet stream troughs
– Occur on order of 1000 km
– Examples in northern U.S. - nor’easters
• Mid-level: individual thunderstorms
– ~10 km
– Higher wind speeds than nor’easters
• Small scale: tornados
– Even higher wind speeds
• Conditions
to get a
large-scale
cyclone
Nor’easters
• Low pressure system moves along NE coast
• CCW circulation draws cold air from north,
moisture from Atlantic
• Works to feed cyclone to produce blizzards,
ice storms
Example: 1993 March “White
Hurricane”
• Covered much of eastern
U.S.
• 100 mph winds, 238 deaths,
over $800 million damage
• Why? Very low pressures,
jet stream trough, wet
western front collide
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
How Low was
the Pressure?
Blizzards
• Winter cyclone
• Characterized by low temperatures, strong
winds (37 mph+), usually snow
• Fatalities come from
– Freezing temps
– Accidents
– Heart attacks
Example: 1996 January
• Record snowfalls in
VA, PA, NJ, WV
• 50 mph winds
• Blizzard and
followup flooding:
187 deaths,
$3billion damage
Example: 1886 March
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85 mph winds in eastern U.S.
Snow in NY - almost 50 inches
Snowdrifts to 20 ft
~400 fatalities
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Ice Storms
• Frozen precipitation melts as it falls, refreezes when
reaches ground
• Problem for objects covered in layers of ice (power
lines, trees)
Common damage
from storms: power
outages
Thunderstorms
• Common in
many parts of
the country
• Produce heavy
rain, thunder,
lightning, hail,
strong winds
Days per year with t-storms
How to Form a Thunderstorm
• Need warm, moist air rising
– Less dense than surrounding air, rises higher
• Passes certain height, condensation occurs
to form cloud
– Gets extra energy from latent heat of
condensation
How to Form a Thunderstorm
• In U.S., good conditions in
– Central, southern U.S.
• Warm moist air from Gulf of Mexico meets cold air
from north
– West-central mountains
• Warm Gulf air meets dry desert air
When and Why of a
Thunderstorm
• Common in late
afternoon, early
evening
– Ground, air
temperatures
are highest
• Build
thundercloud by
continuous
updraft of
warm, moist air
When and Why of a
Thunderstorm
• Continue
updrafts, some
precipitation
begins
• Starts
downward flow
• Most violent
period with rain,
thunder, hail
When and Why of a
Thunderstorm
• Dissipates when cold downdrafts
damp out updrafts needed for fuel
• Cloud evaporates, lighter rain
Microbursts
• Violent downdrafts during t-storm
• Particularly hazardous to planes during
take-off and landing
– Close to ground, can be slammed to runway
• Many instances of fatalities from these
events
Danger from Thunderstorms
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Heavy rain
Flooding
Hail
Lightning
High speed winds
Heavy Rain
• Good side: add to water supplies
• Bad side: happens quickly, high intensity
events
– Can lead to flash floods
Example: Central Texas
• 22 inches in 2.75 hours
• 32 inches in 12 hours
– Flooding killed 215 people
• 1972 storm: 12 inches in 1 hour
– Local river rose ~28 ft in 2 hours
– Flooding killed 17, $18 million damage
Hail
• Semi-spherical layered ice balls falling
during thunderstorms
• Requirements:
– Large thunderstorms
– Upper level cold air (maximum temperature
contrasts)
• Produce strong updrafts needed to keep hail
suspended long enough to add layers of ice, grow in
size
Where are hailstorms common?
Same as t-storms?
NO!
Why? Need upper level cold air
Common in late spring/summer. Migrate northward in
summer
Local Hailstorm
• October 2004
• Tropical air moving northward, mixed with cold front
from north
• 2-3 inch diameter hailstones recovered
– 95 - 120 mph when hitting the ground
• Significant financial impact:
– $25-30 million citywide
Lightning
• Discharge of electricity
• Occur during thunderstorms, common in
summer
• Deaths follow common pattern of
thunderstorm occurrence
Lightning fatalities
Highest in Florida, months of June,
July, August
Electrification of Clouds
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Generally upper level of clouds - + charge
Mid level - - charge
Lower level - mix of charge
Why? Transfer of positive ions from warm
to cold objects in collisions between ice
droplets, water droplets
Upper level +
Mid level Lower level mix
Note positive
charge on high
objects near cloud
negative charges
Lightning Strike
• Unlike charges attract each other
– Negative charges in cloud lead to positive
charges on ground
– Dense on high objects (trees, poles)
• Set up electric potential between cloud and
ground
• When large enough, current flows lightning
Lightning Facts
• Several strokes occur over ~1/2 second
• Travels over 6,000 miles/second
• Can produce very high temperatures
(55,000 F) briefly
– High temps cause air to expand, produce sound
wave (thunder)
Deaths
• ~143 deaths per year in U.S.
• Ways to avoid
– Stay inside house, car, truck
– Outside, move to low place
Winds
• Can be straight-line or rotating (tornadoes)
• Straight-line winds can be 80-100 mph
– Damage structures, trees
– Duration can be hours
Next Time
• Tornadoes