Tornadoes

tornado rapidly rotating wind around center of extreme low pressure, in which rotation reaches the ground.

–N. Hem are c-clkws (from above) –windspeeds 250 mph (up to 300 mph) –>1000/yr US (2004: 1,722) –100 fatalities/yr US (45% mobile homes) –diameter: ft (ave: 330 ft) –60 mph; from SW to NE Ave path 4 mi; few minutes

Ingredients: Warm moist air at surface from south (mT) cold drier air approaching from northwest (cP) : cold front warmer dry air from central Mexico aloft (cT) (6500 ft; 2 km) –Provides inversion cap Dryline (dew point front) between 2 dry air masses and warm moist air from Gulf

These conditions lead to formation of Supercell Thunderstorm Large single-cell thunderstorm with rotating updraft. –Rotating updraft is “mesocyclone” Extremely strong updraft –(150 – 175 mph)

Rotating updraft: 1. surface air rotates horizontally

2. Strong updraft pulls vortex into cloud, becomes a vertical rotating mass of air in south side of supercell (mesocyclone)

Mesocyclone Up to 6 mi. diameter; inside supercell Draws in moist air; energy Rotation speed increases stretches vertically Visible as wall cloud beneath supercell

striations

Supercell tornadoes drop out of wall cloud

Warm, moist air at surface –Southerly wind direction Mid level inversion “cap” layer of stable, warm air Upper level cold, dry air –westerly altitude temperature Sounding profile Air temp Dew pt Warm, moist Conditionally stable Inversion cap; stable Dry, cold Conditionally stable

Role of inversion lid: As surface air heats up (hot afternoon), it punches through lid; allows more air to be vented upward; rises very rapidly in cold, dry air aloft: creates strong updraft

0.1 % thunderstorms become supercells <15% supercells produce tornadoes 50% mesocyclones spawn tornadoes

Inflow under mesocyclone draws air into LOW pressure Condenses into visible cloud (funnel cloud) At ground, picks up debris Mesocyclone stretches vertically, shrinks horizontally (Conservation of Angular Momentum); accelerates circulation

Condensation funnel

Debris cloud

Stages 1.Dust Whirl Dust kicked up from ground Small funnel cloud Little damage

2. Organizing More intense More damage Funnel drops lower

3. Mature Funnel more vertical As wide as it gets Most damage Most intense

4. Shrinking decrease in width funnel tilts narrowing damage path

5. Decay Stretches Rope-like

Don’t always have funnel

Most frequent…… –place (in U.S.) : “tornado alley” –time of year (in U.S.) : March - July (esp. May) –time of day: late afternoon

“tornado alley”

Tornado Outbreaks 6 or more March 12, 2006 May 3-4, 1999 (Oklahoma City) Superoutbreak: –“Terrible Tuesday”: April 3-4, 1974 –16 hours, 148 tornadoes, 13 states, 307 deaths, 6000 injured, $600 million –combined path: 4,181 km.

Fujita scale

In US: 69% F0, F1 29% F2, F3 2% F4, F5

F3 damage

F5 damage

Safety INDOORS: stay away from windows basement; mattress small interior room under stairs

Go to lowest floor Crouch down, cover head Under table, chair In mobile home, get outside, to shelter or low spot, lie flat

OUTDOORS: Stop car Get to sturdy building if nearby Get in ditch or low area Lie flat, away from trees or cars Don’t go under bridges or overpasses

Don’t try to outrun a tornado in a car!

flying debris is a major hazard

Watches and Warnings

TORNADO WATCH Issued when conditions are conducive to tornado development 4 – 6 hours from Storms Prediction Center, Norman OK

TORNADO WARNING Issued if tornado is seen on radar or by an eye witness (spotters look for tornadoes when a watch is issued). from NWS

Radar signature Hook echo : strong rotation in mesocylone; possible tornado

2 hook echos, May