Chapter 12
Hurricanes
Hurricanes = tropical cyclones = typhoons. Convert heat energy of ocean into winds and waves
Hurricanes push surges (large mass of sea water) onshore and raise sea level
Long lasting: about an hour or two; bigger, slower, longer than tornadoes
Hurricanes need:
o Warm water – greater than 27 degrees C threshold. They are a heat engine extracting
energy from warm (ocean) waters. Water must be greater than 60m thick
o No or little wind shear (winds travelling different directions/velocities)
o Air over the water must be warm, humid and unstable (usually around equator)
Running off latent heat of condensation
o Coriolis Effect must occur to spin the system - Air spirals upwind – cyclones, typhoons
o Upper-level winds should be weak and preferably blowing in the same direction the
storm is moving
Hurricanes are unique:
o Latent heat released by condensation inside a hurricane is its main energy source
o Hurricanes weaken rapidly when they move onto land
o Fronts are not associated with hurricanes
o Weaker high-altitude winds = stronger hurricanes
o Hurricane centers are warmer than their surroundings
o Hurricane winds weaken with height
o Air in the center of the eye sinks downward
Hurricane development:
o Tropical disturbance - low pressure zones that draw in a cluster of thunderstorms with
weak winds
o Tropical depression – surface winds strengthen and flow around a center. Converging
surface winds send warm/moist air upwards to stratosphere where it condenses and
releases latent heat. This warms surrounding air, and strengthens updrafts
o Tropical storm – surface wind speed exceeds 63 km/hr
o Hurricane – surface wind speed exceeds 119 km/hr
Figure 12.12 – troposphere; northern hemisphere counterclockwise rotation by virtue of the
Coriolis Effect. Air spirals upwards; convection is powered solely by the latent heat of
condensation
Hurricanes don’t form on the actual equator (because it’s Coriolis Effect is zero), and do not
cross the equator. Form near the equator. Commonly off the Pacific Coast of Mexico, rarely off
of Brazil
Wind speeds: 320km/hr is fastest, tornadoes are faster.
Diameter is approx 1000km, core where it’s fastest is >200km.
Depending which side of the storm you’re on will determine how much damage occurs. On the
side where wind speed goes in the same direction as the storm motion, damage will be greater
than opposite side where wind speed goes in opposite direction from the storm motion
o Damage is asymmetrical in storms
o In the northern hemisphere, right side will experience speed of the storm body plus
winds (wind comes from ocean), on the left you will feel the wind speed minus storm
motion (wind comes from land) Eye: calm area 30-60 km in the middle of the hurricane, no winds can reach this center when
wind speeds exceed 119 km/hr. Cool air sinks, leaving eye clear/cloud free. Eye walls contain
fastest winds
Energy released by a hurricane by forming clouds and rain is 400x greater than the energy of its
winds
Saffir-simpson scale measure categories of hurricanes
Post tropical transition: when some hurricanes unexpectedly gain strength by moving to higher
latitudes and hit cooler/stronger air streams. Hurricane structure deforms and becomes less
compact
Surge: when the sea comes ashore. Drowning causes most hurricane fatalities. Sea level rises
because water build-up beneath the eye because it’s a low pressure zone, and winds on the side
of the storm where winds are additive push sea swells ashore.
Fatalities caused by:
o Majority is wind
o Hydrostatic response to pressure
Water coming ashore is more deadly than wind
Figure 12.22/12.27
North Atlantic Ocean experiences 4-28 hurricanes a year. Late summer is greatest risk because
the water retains the most energy from the whole summer (Aug-Oct)
Hurricane seasons: early hurricanes stay in the Caribbean, or enter the Gulf of Mexico. Later in
the season hurricanes are less likely in the gulf, more likely around the eastern coast of the US.
Hurricane status usually revoked by the time storms hit Canada. Luis is a notable exception cat 3
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