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. Frozen particles are always heavier than the cloud they are in

2. Warm air can carry frozen particles with it

3. Cold air always flushes frozen particles completely out of the cloud

4. Hail should be more common than rain, since the particles are frozen in the cloud

5. 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

http://ozreport.com/pub/images/supercell.jpg&imgrefurl

Rotating mesocyclone

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 cone- or 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 41%

Permanent homesexterior rooms w/windows

30%

Car/truck/busfar fewer in small cars

10%

School/church 6%

Outdoors 5%

Business 5%

“other” 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.