Hurricanes
(Tropical Cyclones)
Geography of storm names
Atlantic and eastern Pacific: “hurricanes”
Indian Ocean and Australia: “cyclones”
Western Pacific, N. hemisphere: “typhoons”
Western Pacific typhoons, 2011
Season: June 1 – November 30
Eastern Pacific hurricanes
Hurricane Linda
Season: May 15 – November 30
Atlantic hurricanes
Season: June 1 – November 30
Australian cyclones
2010-2011 season
Season: November 1 – April 30
North Indian Ocean cyclones
Season: April – December
South West Indian Ocean cyclones
Season: November 15 – April 30
Origin regions:
 Requirements:
 expansive surfaces of warm (>26⁰C), deep ocean waters
 no upper level wind shear
 Coriolis (therefore do not develop at the equator)
Trade wind Inversion
 In an INVERSION, temperature
increases with altitude
 Instead of the normal decrease with altitude
 Recall the Hadley Cell
circulation:
 Descending air in
subtropical Highs
 Creates a subsidence
inversion
 Subsiding air warms
 Makes a layer of warm air
over cold air
Subsidence inversion
Inversion is strongest at
Subtropical highs and
weakens towards equator
Inversion prevents
tall clouds from forming
Trade winds
equator
Marine layer
(beneath inversion)
subtropics
Inversion is also lowest along eastern margins of oceans ,
Where upwelling and cold, ocean currents keep the marine layer cool.
Inhibits vertical cloud growth.
On western side of ocean basins, warm surface waters heat marine layer and
it expands to a greater height . Tall cloud growth is possible.
Therefore, hurricanes form in western portions of ocean basins.
Ways that hurricanes begin:
1. as easterly waves
(Hurricane Georges is an example)
Requires spin
Most Atlantic hurricanes begin in Africa
Pacific High
Azores Bermuda High
Monsoonal Low
NE Trades
ITCZ
Turned SW by Coriolis deflection
equator
ocean
Africa
SE Trades
If storm moves off African continent towards
Atlantic, becomes “easterly wave” (carried west
by equatorial easterlies)
Of approx. 100 each year, 6 become
hurricanes
60% of the Atlantic tropical storms and
minor hurricanes originate from easterly
waves
nearly 85% of major hurricanes have their
origins as easterly waves
nearly all of the tropical cyclones that occur
in the Eastern Pacific Ocean can be traced
back to Africa
2. with wind shifts associated with fronts in the
tropics
(Hurricane Arlene is an example)
NE trades
front
S winds
front
Easterly wave moves across Atlantic as a small bunch of thunderstorms;
May become a tropical disturbance : disorganized group of thunderstorms
that spin
Progression of storms to hurricane:
1. tropical disturbances
disorganized groups of t-storms; some spin
90% fizzle out
2. tropical depressions : wind speeds 23 mph
3. tropical storm : wind speeds 39 mph (named)
4. hurricane : wind speed 74 mph
Speed and direction
in North Atlantic basin:
 10 – 20 mph travel (NOT wind speed)
 Originally steered by easterly winds (trades)
 Are drawn into circulation around subtropical highs and are
veered to the north (clockwise)
 When north enough, are steered by westerlies
 Details of paths are erratic
 Due to storm structure and interaction with environment
Anatomy of a hurricane
Spiral bands of
thunderstorms
Strong central LOW
Convergence at surface
Moist warm air rising
Condensation
Upper level winds and
cirrus clouds at top
Rising air causes
crowding at top center
Eye and Eyewall
Causes a HIGH at top
center
HIGH at top and LOW
at bottom makes air
descend in center
Descending air can’t
make clouds
Therefore clear EYE
in center
 Eye is clear, calm
 Average : 15 miles wide; experienced on ground for approx. 1 hour
 Eyewall: surrounds eye
eyewall
 Greatest wind speed, thickest cloud cover; heaviest rain (100
in/day)
 Wind speeds strongest here due to conservation of angular
momentum
Hurricane hunter air flight