HURRICANES
Vicki Drake
Department of Earth Sciences
Santa Monica College
I. Hurricane
A. Largest of all mid-latitude cyclones
1. 200-700 km in diameter (150-350 miles across)
B. Most destructive of all storm systems
C. Formation
1. Form over water – not land
2. Need warm tropical waters (sst~250C{80-850F})
a. Release of latent heat from high humidity warm air
3. Latitudes - ~5-250N/S of Equator
a. Not at equator – no Coriolis Force (rotate CCW)
b. InterTropical Convergence Zone – convergence of
Surface air masses
c. Light surface winds
d. Follow ‘path’ of Trade Winds – pushed “westerly” over tropical waters
4. Typical Season – June through November (N. Hemi Summer)
5. May or may not occur along fronts – not necessary
6. Cold upper air trough with diverging air aloft to enhance
Development of surface low
a. Extremely low barometric pressure (~965 mb)
7. Tropical Thunderstorm development and organization
D. Dissipation
1. Trade Wind Inversion- no hurricane formation
2. Travel over cold water = loss of ‘fuel’ (latent heat)
3.Travel over land – loss of ‘fuel’ and friction of land
E. Thunderstorm Organization
1. Latent heat release fuels thunderstorm growth
2. Thunderstorms organize about a central low pressure cell
3. Unstable air aloft allows towering Cb clouds to form
F. Energy
1. Latent heat release
2. Warm air rising – upper level rise in air pressure
3. Diverging air aloft – enhanced surface Low pressure
4. CCW (counterclockwise) spin about low and into the center (Cyclonic flow)
5. Winds create rough seas and turbulence to release more ascending air into
thunderstorm development
G. Feedback Mechanism
1. Rising warm, humidity air picks up warm ocean water – adding more fuel to
system
2. Upper level outflow of air >surface inflow – intensification of storm with
surface air pressure dropping
H. Interior “Schematics”
1. Towering Cb Thunderstorms – rising air columns
2. Falling air columns between Cb clouds
3. Rotation/Spin about Central Low Pressure Cell
4. Eye Wall – area of greatest precipitation and winds – Ring of thunderstorms
circling the “Eye” of hurricane
© Vicki Drake
SMC F-2000
1
5. Eye – falling air column over central Low Pressure – Warm, descending air,
calm, generally cloud-free, precipitation-free, wind-free, - but pressure is
lowest at the very center of the Eye
6. Storm Surge – Extremely high waves of ocean water created by turbulence of
storm,
a. High winds and high water
b. Inundating low-lying coastal areas for miles inland
c. If coupled with high tide, may create waves over 33 feet tall
I. Movement
1. Forms in tropical waters except very South Atlantic and Eastern South Pacific
a. Waters too cold
2. In North Pacific and North Atlantic – Trades Winds push storm along to the
west
3. At subtropical High Pressure Zone – the storm tends to swing North and get
caught up in the Westerlies (now moving to the east)
4. Forward speed increases
5. Fastest winds on the approaching storm side– slowest winds on the
“retreating” storm side
J. Stages of Development
1. Tropical Disturbance – Thunderstorms beginning organization, slight circulation
about a central low pressure
2. Tropical Depression – Wind speed of 20-34 knots, closed isobars at the very
center of storm
3. Tropical Storm – 35-64 knots, tight isobars (indicating a very low barometric
pressure), CCW rotation of air about the center, traveling movement across
oceans
4. Hurricane - >64 knot wind speeds, lowest pressure in center even with
descending air, pronounced “eye” develops, movement across oceans
K. Naming
1. Originally – identified by latitude and longitude only
2. Military code names (Able, Baker, Charlie, etc.)
3. 1953 – Female names for both Pacific and Atlantic hurricanes
4. 1978 – male and female names used
L. Saffir-Simpson Hurricane Damage-Potential Scale
Level 1; ≥980 mb; 74-95 mph winds; shrubbery/tree damage
Level 2; 965-979 mb; 96-110 mph winds; trees, mobile homes, roof damage
Level 3; 945-964 mb; 111-130 mph winds; trees, mobile homes, structures
Level 4; 920-944 mb; 131-155 mph winds; trees,mobile homes,structures,flooding
Level 5; <920 mb; >155 mph winds; total destruction low-lying areas/structures
II. Differences between Hurricane and “Standard” Mid-Latitude Cyclone
A.Energy
1. Hurricane – warm sea water and release of latent heat
2. Storms – temperature differences between contrasting air masses and release
of latent heat
B. Intensity
1. Hurricane – most intense over water, weakens at higher altitudes
2. Storms –intense over land and water, stronger at higher altitudes
C. Center Cores/Pressure
1. Hurricane – central column of warm air, descending pressure: distinctive “eye”
2. Storms – central column of cool/cold air, low pressure; not distinctive “eye”
D. Formation
1. Hurricane – over water, not along fronts, weak winds, warm humid air
2. Storms – over land/water, along fronts, strong winds, rising air
© Vicki Drake
SMC F-2000
2