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: 300 - 2000 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

altit

ude

temperature

Sounding profile

Air temp

Dew pt

Warm, moistConditionally stable

Inversion cap; stable

Dry, coldConditionally 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. OrganizingMore intenseMore damageFunnel drops lower

3. Mature

Funnel more verticalAs wide as it getsMost damageMost intense

4. Shrinking

decrease in widthfunnel tiltsnarrowing damage

path

5. Decay

StretchesRope-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, F129% F2, F32% F4, F5

F3 damage

F5 damage

F5 damage

Safety

INDOORS:stay away from windowsbasement; mattresssmall interior roomunder 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 carGet to sturdy building if nearbyGet in ditch or low areaLie flat, away from trees or carsDon’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 3 1999