Lecture 27. Hurricanes.
Stages of hurricane development. Structure
of a hurricane. Hurricane formation.
Destruction caused by hurricanes.
Reading: Ch. 11.
Definition
• A hurricane is an intense tropical storm with
winds exceeding 75 mph
• “Official”name for a hurricane —
tropical cyclone
• Tropical cyclones have a low-pressure center,
and surface winds rotating counterclockwise
in the Northern Hemisphere and clockwise in
the Southern Hemisphere
Tropical waves
• streamlines on a tropical
weather map depict wind flow
• a kink in streamlines represents
a weak trough of low pressure
— tropical wave (or easterly wave)
• tropical waves move from
east to west and leave an
area of showers behind
• tropical waves may intensify
and grow into a hurricane, if conditions are right
1
Stages of hurricane development
• Defined based on the wind speeds in
a tropical cyclone:
 tropical disturbance*
 tropical depression (23–38mph)
 tropical storm (39–74mph)
 hurricane (>75mph)
* !No closed isobars, difficult to detect
Structure of a hurricane
• eye — the center of the hurricane (40–50km wide):
lowest pressure, broken clouds and light wind
• eye wall — ring of intense thunderstorms: heaviest rains and
strongest winds
(wall height ~ 15 km)
• a series of alternating spiral rain bands and rain-free areas
• circular structure, no fronts
• ~500 km in diameter —
smaller than a mid-latitude
cyclone
Vertical cross-section through
a hurricane
• The vertical structure of a
hurricane is much different
from that of a mid-latitude
storm
• release of latent heat as warm,
moist rising air cools in the eye wall
produces warm air and increasing
pressure aloft, initiating sinking
within the eye of the storm
2
Hurricane formation
• Necessary ingredients:
 Warm surface waters over a large area
 Convergence of surface winds
• The presence of these conditions does not
guarantee a hurricane formation. Other factors:
strength of upper-level winds (wind shear)
 interaction with mid-latitude weather features
! Hurricanes get their energy from the latent heat of condensation.
Latent heat release due to rising air in the eye wall results
in warmer air and higher pressure aloft, increased upper-air
divergence, decreased surface pressure, and more of rising air
Hurricanes dissipate
• as soon as the source of latent heat
is cut off, for example:
 hurricane moves over colder water
 hurricane moves over land
 surface inflow of air exceeds upper-level
outflow of air
Patterns of hurricane
movement
• Hurricanes are guided:
 initially by trade winds
 then by air flows around
semipermanent highs
 later by mid-latitude westerlies
 by local surface conditions
3
Destruction caused by hurricanes
Destructive elements
of hurricanes:
 high winds
 extremely heavy precipitation
 increased ocean levels (storm surge)
 tornados
Hurricane winds
! If a hurricane is approaching from the east,
the highest winds will be on the north side
of the hurricane (winds that translate the
hurricane will add up there to clockwise rotation)
Storm surge
• abnormal rise of ocean level by several meters due to
combined effect of storm’s low pressure, storm-wind-induced
upwelling, and, maybe, high tide
• causes most of the destruction in coastal regions
4