Severe Weather

ThunderstormsTornadoesHurricanes

Thunderstorm formation

• Formed by strong upward movement of warm, unstable, moist air

• Cumulus clouds form single cloud• Grow vertically• Include lightning & thunder• Usually produce rain

Storm over Mississippi

Winds in thunderstorm

• Heating of the ground causes vertical motion of air

• Weight of falling rain & cooling by evaporation cause downdrafts

• Downdraft spreads, choking off the vertical motion

• Cloud dies

Types of Thunderstorms

• Local– Mostly occur in spring or summer– Widely scattered– Photo: lake tahoe

Types of Thunderstorms

• Frontal–Often occur in lines along frontal

surface–Squall line = formed ahead of front–Occur most often in spring &

summer–May last several hours

Lightning

• Discharge of electricity• + at top; - at bottom of cloud• Temp of lightning flash ~ 28,000° C• Light travels 300,000 km / second• Sound travels .3 km / second• So… we see flash before rumble

Thunderstorm Safety

• Get indoors!• If you get stuck outside…crouch on the

ground…do NOT stand under a tree

Tornado

• Narrow, funnel-shaped column of spiral winds that extends downward from the cloud base

• Touches the ground

Conditions

• Strongest winds 360 – 800 km/hr• Winds move counterclockwise• Travels at speeds of 40 – 65 km/hr• Typically last an hour maximum• Accompanied by heavy rain,

lightning & hail

Tornado formation

• Warm, moist air from the Gulf of Mexico moves N

• Higher up, cool air moves east over Rocky Mtns.

Tornado Safety

• Get indoors– Basement, interior rooms, bathroom or closet

• Outdoors? Find a ditch & lie flat & protect head

Hurricane

• Intense tropical low-pressure area• Sustained winds of 120 km/hr• Photo: hurricane Isabel, Oct 2003

Hurricane formation

Hurricane safety

• Heed warnings!• Be prepared with radio, batteries, canned food

& medicine

Storm surge

• Greatest damage associated with hurricanes is caused by the storm surge (Katrina in New Orleans)

• Water piles up & winds blow it inland