11C-1 Thunderstorms 40,000 each day on earth Most common violent storm Small (few km in diameter) Form on hot summer days due to

convection or on the edges of cold fronts

Thunderstorms

Thunderhead– Cumulonimbus cloud (vertically

developed)– Anvil shaped on top

Thunderstorms

Lightning: Electrical discharge– Cloud to cloud– Cloud to ground– Top of cloud becomes + & bottom

becomes –– Voltage difference causes spark to jump

Stepped leader– Electricity heading down in steps

Return strokes– From ground up to stepped leader– Happens so fast it looks like 1 flash

Forked lightning– If stepped leader branches on the way

to the ground

Thunderstorms

Temperature of lightning bolt is very hot

Forces air nearby to expand rapidly, causing thunder sound

Speed of light & sound not the same– Count seconds between flash & thunder

to figure out distance (& ÷ by 3)– Sound travels 1/3 km per second– If 15 seconds, 15/3 = 5 km away

Can’t hear thunder beyond 16 km

Thunderstorms

Heat lightning– Same as regular lightning, but too far

away to hear thunder

11C-2 Tornadoes

Narrow funnel cloud extending from cumulonimbus cloud

Narrow path of destruction Highest speeds of any storm (515

km/hr) Occur all over the world, but USA has

the greatest number & most destructive

Waterspout– Tornado at sea (less intense)

Tornado Base– The part on the ground– Most destructive part– May be 1 m to 5 km (400 m average)

Fujita Pearson Scale based on…– Windspeed (estimated from damage)– Path length– Path width– F5 is highest

Tornado

Can have several funnels from same cloud

Can travel 100 km/hr on ground Often zigzags General SW NE direction due to

prevailing westerlies Destructive forces:

– Windspeed & updraft– Can lift cars, houses, etc

11C-3 Hurricanes Huge cyclonic windstorms that form

over tropical or subtropical oceans Huge diameter (160 km to 1000 km) 119-320 km/hr winds Dangerous to ships & shorelines due

to…– 1) High winds– 2) High tides– 3) Heavy rains/flooding

Typhoons – hurricanes in Pacific region

Cyclones – hurricanes in Indian Ocean

Tropical cyclones– Form near equator– Small & violent

Extra tropical cyclones– Form over middle latitudes– Larger but less violent

Hurricanes Require warm water

to form Usually 10-15° latitude

How hurricanes form– Warm, moist air rises in a low pressure

area– Cool air rushes to fill the low pressure

area– Spiral forms which funnels more air

which is moist & rises more spiral of winds

– Called a hurricane when >119 km/hr winds (tropical storm if slower)

Saffir-Simpson Hurricane Categories: 1-5

Eye of the hurricane– In the center of the spiral– Low pressure– Can have calm air,

no rain

Hurricane over land…

Slows down Loses energy due to friction with land

and no longer a supply of warm moist air

Wind slows no longer a hurricane Heavy rains continue

Storm swell– Huge waves produced

by the high winds– Hits shore before

hurricane arrives Storm surge

– Increase in sea level along shore as hurricane winds pile up water against the land

– Can cause severe flooding (esp. when added to high tide)

Hurricane Safety

Hurricanes cause loss of life & property

Safest to evacuate Hurricanes seen in

advance on satellite– Good tracking systems &

early warning systems help, but path may still be unpredictable

Facet: Safety in storms

Thunderstorms– Safest place: inside a building with a lightning

rod Special metal rod on the building that goes down

into ground, attracts lighting (lighting goes through rod not building)

– Next safest: indoors not touching metal, pipes, phones, computers

– Next safest: in a car (sit in middle, don’t touch doors)

– If outside: avoid tall objects/trees, don’t be highest in object in a field (crouch don’t lie down)

Tornadoes: Best protectionis storm cellar – Next best: basement (get under strong

furniture along southwest wall)– Next best: small windowless room like

closet/bathroom– If outdoors: don’t try to outrun tornado, find

lowest area (lie in a ditch), car under overpass is better than out in the open (stay in car)

Facet: Safety in storms

Hurricane: evacuate the area– Board up windows– Leave if instructed by authorities to do

so

Facet: Safety in storms