HURRICANES

• also called typhoons (Pacific), willi willis (Australia), tropical cyclones

hurricane formation• form at 15 to 20o N or S of equator – not at

equator (Coriolis too weak)• must have warm water and humid air to

form• low pressure

cells (tropicaldepressions)strengthen andspeed up

tropical depressions coming off of Africa

hurricane formation

• Low pressure cell moves in• Air warmed by warm ocean• Water evaporates• Warm air rises and spirals due to

Coriolis effect• Forms column of warm air rising

hurricane formation

• Since low pressure, more air rises• As warm, moist air cools, it

condenses and rains• Keeps strengthening as long as

over warm water

hurricane structure

• Doughnut shaped clouds with hole (eye) in middle

• Can be few hundred miles diameter, 9 miles high• Eye – up to 20 miles wide– Calm– Warm air rises due to low pressure– Cold, dry air sinking – prevents rain

• Warm air exits top and spirals out due to Coriolis

hurricane structure

• Northern hemisphere – spiral counter-clockwise

• Southern hemisphere – spiral clockwise• Blown across ocean by trade winds – affected

by Coriolis, so move north or south – move 3 to 25 miles/hour– In Atlantic move up coast from Carribean

• tropical depression – winds < 38 mi/hr• tropical storm – winds between 39 and

73 mi/hr• hurricane – winds > 74 mi/hr

hurricane strength

hurricane demise• If upper level winds strong can sheer off

top of hurricane causing it to break up• If over cool water cannot maintain itself• If over land cannot maintain itself• Atlantic Hurricane season is June to

November, but may occur other times

the power of hurricanes

• Release huge amounts of energy–More than U.S. energy needs for 1

year• Up to 20 billion metric tons of rainfall–> I inch per hour

• Can be extremely damaging

hurricane damage

• Wind damage• Rainfall = flooding• Storm surge – especially if hits

land at high tide– Ex: Bangladesh 1970 had a 40

foot storm surge