Hurricanes form over warm ocean waters between 15 and 200 latitude from the equator when low pressure systems allow warm, humid air to rise and condense. They have a circular structure with counterclockwise spinning winds in the Northern Hemisphere that can be hundreds of miles wide. Hurricanes strengthen over warm ocean waters but weaken when making landfall or interacting with strong upper level winds or cooler waters. They can cause extensive wind damage, flooding from heavy rains, and storm surges that become especially destructive during high tides.

1. Hurricanes

2. Other Names
• Typhoons (in the Pacific)
• Willi willis (in Australia)
• Tropical cyclones

3. Hurricane Formation
• Hurricanes tend to form at 15 - 20⁰ N or S of
equator (Not at equator since Coriolis is weak
there)
• Require warm, humid air
• Are low pressure cells (tropical depressions)
• Strengthen and speed up as they travel from origin

4. Tropical Depressions Coming off Africa

5. Hurricane Formation cont.
• Low pressure cell moves in
• Air warmed up by the warm ocean
• Water evaporates
• Warm air rises & spirals due to Coriolis
• A column of warm air forms & rises

6. Hurricane Formation cont.
• Pressure is low,
allowing warm air
to rise
• As warm, moist air
cools it condenses
& rains
• Hurricane can keep
strengthening as
long as it’s over
warm water

9. Hurricane Structure
• Donut shaped clouds with a hole (the eye) in
the middle
• Can be a few hundred miles in diameter &
nine miles high
• Eye – can be up to 20 miles wide
Calm
Warm air rises due to low pressure
Cold dry air sinking prevents rain
• Warm air exits top & spirals out due to Coriolis

10. Hurricane Structure cont.
• Northern Hemisphere – spiral counterclockwise
• Southern Hemisphere – spiral clockwise
• Blown across ocean by Tradewinds, but affected by
Coriolis, so move North and South at 3-25 mph
-In the Atlantic,
they move up from
the Caribbean

12. Hurricane Strength
• Tropical Depression – winds > 38 mph
• Tropical Storm – winds between 39 – 73 mph
• Hurricane Winds - >74 mph

15. The Demise of the Hurricane
• If upper level winds are strong they can sheer off
the top of the hurricane & cause it to break up
• Hurricanes over cool water can’t sustain themselves
• Hurricanes over land can’t sustain themselves (but
can lead to the development of tornadoes inland)
• Atlantic hurricane season is June – December (but
they can rarely occur at other times)

16. The Power of Hurricanes
• They can be
extremely damaging
• They release HUGE
amounts of energy
– more than the US
needs in an entire
year
• They release up to 20
billion metric tons of
rain
– > 1 inch per hour

17. Types of Damage Typical of Hurricanes
• Wind damage
• Flooding from all that rain
• Storm surge damage, especially if it hits during high
tide and/or spring tide
– Ex: Bangladesh (1970) had a 40 foot storm surge
– Ex Massapequa (2012) had a 15 foot storm surge
• (5 feet above my in-laws basement and into the 1st floor)