Severe Weather

Flash Floods•Flash floods occur when rainfall or snowmelt exceeds the lands infiltration rate and/or the carrying capacity of a river.•Flash floods can occur in desert landscapes, valleys, coastal areas, and flood plains along rivers. •Floods kill more people than any other disaster.

Thunderstorms• Characterized by: Dark cumulonimbus clouds that

produce heavy rain, sounds of thunder, flashes of lightning, strong winds, and sometimes hail.

Thunderstorms often occur in the summer when the air is warm, moist, and unstable.

Where do thunderstorms form?• Thunderstorms form along fronts because air

is forced to rise and ahead of fronts called squall lines. Supercells are large single-cell thunderstorms with extremely strong updrafts, the updrafts rotate to form tornadoes.

How does hail form?

• A tornado is a violently rotating column of air that forms from a mesocyclone, when it touches the ground it becomes a tornado. Air pressure on the center of a tornado is very low and air is sucked into the funnel cloud.

Where is “tornado alley”?• A tornado can form in any state during any time of

year, but they are most common in “tornado alley” during the spring and summer months.

A tornado is measured using the Fujita scale, it is based on wind speed.

Hurricanes• A hurricane starts

out as a tropical storm. It begins to gain strength from warm ocean water, which evaporates and then condenses to make storm clouds. Winds help push warm, moist air upwards and help the hurricane get bigger as it rotates and moves.

Hurricane diagram• The coriolis

effect causes a hurricane to rotate counter-clockwise.

A hurricane in the South Pacific is called a typhoon.

• A hurricanes strength is measured using the Saffir-Simpson Scale, it is based in wind speeds, category 1-5.

***Notice how air pressure decreases when the wind speeds and storm surge increases?

Blizzards• The winds must exceed 35 mph or 56 kmph.• The temperature must be below 19°F or -7°C.• The blowing snow must reduce visibility by

less than ¼ of a mile.• Conditions must persist for more than 3

hours.