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today
• Tornadoes
• Some review?
Clickers please
Thunderstorms form from
1. rapid updraft of warm, moist air that cools
quickly
2. rapid downdraft of cold, dry air that warms
quickly
3. slow updraft of warm, moist air, so slow
cooling
4. rapid updraft of warm, moist air that cools
slowly
Thunderheads form the classic
“anvil” shape because
1. rising cold air spreads as it reaches the
tropopause
2. rising warm air moves up into the
stratosphere, which is an extremely thin
layer and so compresses the warm air
3. rising warm air spreads as it reaches the
tropopause
4. the mix of warm and cold air high in the
cloud causes the upper edge to flatten
Which of these is true with
respect to forming hail?
1.
2.
3.
4.
5.
Frozen particles are always heavier than the cloud
they are in
Warm air can carry frozen particles with it
Cold air always flushes frozen particles completely
out of the cloud
Hail should be more common than rain, since the
particles are frozen in the cloud
Hail is never larger than about dime size
Tornadoes…
• “Nature’s most violent storms”
• How do they form?
– the classic answer:
"warm moist Gulf [of Mexico] air meets cold
Canadian air and dry air from the Rockies"
tornadoes may be spawned
from thunderstorms but differ
in a critical way
Central US tornadoes
• Warm, moist air from the Gulf of Mexico at
low altitude; ~75oF on the ground
• Middle-altitude, cold air mass moving from
Canada
• High-altitude jet stream moving ~ W to E
The critical factor is wind shear -- areas of
very different air pressure moving
quickly along side each other (spin)
Building a tornado
• Supercell thunderstorm: differs from
“normal” (single cell) storm
– rotating updraft
– tends to become tilted downwind
– huge rotating mass in the center of the
cloud called a mesocyclone
Organizational stage
• rising warm air masses are sheared by
mid-level colder air to form
mesocyclone
• tilt allows warm air to rise in center,
cooler air falls in front (rain/hail) and
cooler, drier air in back
Rotating mesocyclone
http://ozreport.com/pub/images/supercell.jpg&imgrefurl
Organizational stage
Downdrafts in
the trailing side
of the cloud
cause wall
cloud
www.uwm.edu/~kahl/images/weather/clouds/convective/wallcloud.jpg
Organizational stage
• Wall cloud rotates, rotating core pulls
into tighter spiral, speed increases
• Funnel cloud descends
• Dust/debris on ground start to swirl
(dustdevil)
• If the two connect: tornado
Mature stage
• Funnel extends from ground to storm
• Downward-moving air in core is
surrounded by upward-spinning coneor cylinder-shape funnel: giant vacuum
cleaner
www.yorkville.k12.il.us/webquests/webqlanchance/images/tornado
Mature stage
• Highest speeds ~ 100 m above ground
level
• Ground speeds slowed by cars/trees/
houses/ground/etc
• Most common in April/May/June/±July
as winter weather patterns diminish and
spring/summer patterns take over
Mature stage
• Weak tornadoes
–
–
–
–
>85% of all tornadoes
lifetime 1-10 minutes
winds <110 mph
<5% of tornado deaths
• Strong tornadoes
–
–
–
–
~10% of all tornadoes
lifetime 20 minutes or longer
winds 110-205 mph
30% of tornado deaths
• Violent tornadoes
–
–
–
–
<1% of all tornadoes
lifetime >1 hour
Winds >205 mph
70% of tornado deaths
Shrinking stage
• Supply of warm air diminishes
• Funnel width decreases, winds increase
Rope stage
• Erratic movement of thinning cloud
www.stormeffects.com/images/52414-ChesterNEtor8arope.jpg
Fujita scale (analogous to
modified Mercalli)
Highest wind speed I found: 318 mph (~500 kph), Oklahoma City, May, 1999
US deaths from tornadoes
Mobile homes
Permanent homes
41%
30%
exterior rooms w/windows
Car/truck/bus
10%
far fewer in small cars
School/church
Outdoors
Business
“other”
6%
5%
5%
3%
Risk: acceptable vs
unacceptable
• Where are you from (where have you
spent most of your life)?
• What is the major hazard there (eq,
volcano, tornado, excessive heat or cold
[can kill people])?
Risk: acceptable vs
unacceptable
• Building a nuclear-power plant on the
San Andreas fault
• Farming in the Toutle River Valley on
the north slope of Mount St. Helens
• Putting a mobile-home park in
Oklahoma City.