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