Download TABLE OF CONTENTS

BENZING & ROSS, LLC
COMMUNITY EMERGENCY PLANNING & RESPONSE
TABLE OF CONTENTS
Page
HURRICANES
DEFINITION
PARTS
HOW HURRICANES FORM
SIZE, LOCATION & DAMAGES
WEATHER ALERTS
HOW MUCH ENERGY DOES A HURRICANE RELEASE
CONVERSION FACTORS
1
1
2
2-3
4
4
5
6
Hurricanes
Every year between June 1 and November 30 (commonly and called
hurricane season), hurricanes threaten the easterngulf coasts of
the United States, Mexico, Central America and the Caribbean. In
other parts of the world, the same types of storms are called
typhoons or cyclones. Hurricanes wreak havoc when they make
landfall, and they can kill thousands of people and cause billions of
dollars of property damage when they hit heavily populated areas.
Defining a Hurricane
According to the National Hurricane Center, "hurricane" is a name for
a tropical cyclone that occurs in the Atlantic Ocean. "Tropical
cyclone" is the generic term used for low-pressure systems that
develop in the tropics. "Tropical cyclones with maximum sustained
surface winds of less than 17 meters per second (39 mph / 62.7 kph /
34 knots) are called tropical depressions. Once the tropical cyclone
reaches winds of at least 17 meters per second (m/s), it is typically
called a tropical storm and assigned a name. If winds reach 33 m/s
(74 mph / 119 kph / 64 kt), then it is called a "hurricane."
1 of 1
BENZING & ROSS, LLC
COMMUNITY EMERGENCY PLANNING & RESPONSE
Parts of a Hurricane
Once a hurricane forms, it has three main parts:
Eye - the low pressure, calm center of circulation
Eye wall - area around the eye with the fastest, most violent
winds
Rain bands - bands of thunderstorms circulating outward from
the eye that are part of the evaporation/condensation cycle that
feeds the storm
How a Hurricane Forms
Hurricanes form in tropical regions where there is warm water (at
least 80 degrees Fahrenheit / 27 degrees Celsius), moist air and
converging equatorial winds. Most Atlantic hurricanes begin off the
west coast of Africa, starting as thunderstorms that move out over the
warm, tropical ocean waters. A thunderstorm reaches hurricane
status in three stages:
Tropical depression - swirling clouds and rain with wind
speeds of less than 38 mph (61.15 kph / 33 kt)
• Tropical storm - wind speeds of 39 to 73 mph (54.7 to117.5
kph / 34 to 63 kt)
• Hurricane - wind speeds greater than 74 mph (119 kph / 64 kt)
It can take anywhere from hours to several days for a thunderstorm to
develop into a hurricane. Although the whole process of hurricane
formation is not entirely understood, three events must happen for
hurricanes to form:
• A continuing evaporation-condensation cycle of warm, humid
ocean air
Patterns of wind characterized by converging winds at the
surface and strong, uniform-speed winds at higher altitudes
A difference in air pressure (pressure gradient) between the
surface and high altitude
Warm, moist air from the ocean surface begins to rise rapidly. As this
warm air rises, its water vapor condenses to form storm clouds and
droplets of rain. The condensation releases heat called latent heat of
condensation. This latent heat warms the cool air aloft, thereby
2 of 2
BENZING & ROSS, LLC
COMMUNITY EMERGENCY PLANNING & RESPONSE
causing it to rise. This rising air is replaced by more warm, humid air
from the ocean below. This cycle continues, drawing more warm,
moist air into the developing storm and continuously moving heat
from the surface to the atmosphere. This exchange of heat from the
surface creates a pattern of wind that circulates around a center.
"Converging winds" are winds moving in different directions that run
into each other. Converging winds at the surface collide and push
warm, moist air upward. This rising air reinforces the air that is
already rising from the surface, so the circulation and wind speeds of
the storm increase. In the meantime, strong winds blowing at uniform
speeds at higher altitudes (up to 30,000 ft / 9,000 m) help to remove
the rising hot air from the storm's center, maintaining a continual
movement of warm air from the surface and keeping the storm
organized. If the high-altitude winds do not blow at the same speed at
all levels -- if wind shears are present – the storm loses organization
and weakens. High-pressure air in the upper atmosphere (above
30,000 ft / 9,000 m) over the storm's center also removes heat from
the rising air, further driving the air cycle and the hurricane's growth.
As high-pressure air is sucked into the low-pressure center of the
storm, wind speeds increase.
The right side of a hurricane packs more punch because the
wind speed and the hurricane speed-of-motion are complimentary
there. On the left side, the hurricane's speed of motion subtracts from
the wind speed. This combination of winds, rain and flooding can
level a coastal town and cause significant damage to cities far from
the coast. In 1996, Hurricane Fran swept 150 miles (241 km) inland
to hit Raleigh, N.C. Tens of thousands of homes were damaged or
destroyed, millions of trees fell, power was out for weeks in some
areas and the total damage was measured in the billions of dollars.
“It should be noted that 90% of all hurricane related fatalities
result from drowning.”
3 of 3
BENZING & ROSS, LLC
COMMUNITY EMERGENCY PLANNING & RESPONSE
Size, Location and Damages
Hurricanes vary widely in physical size. Some storms are very
compact and have only a few trailing bands of wind and rain behind
them. Other storms are looser, so the bands of wind and rain spread
out over hundreds or thousands of miles. Hurricane Floyd, which hit
the eastern United States in September 1999, was felt from the
Caribbean islands to New England.
Weather Alerts
There are four weather alerts for tropical storms and hurricanes.
Depending on where you are located in proximity to the storm, you
may find yourself under one of these alerts:
A tropical-storm watch is issued when sustained winds from
39 to 73 mph (54.7 to 117.48 kph / 34 to 63 kt) are possible in
your area within 36 hours.
A tropical-storm warning indicates that these conditions are
likely in your area within 24 hours.
A hurricane watch is issued when hurricane conditions
(sustained winds greater than 74 mph / 119 kph / 64 kt) are
possible in your area within 36 hours.
A hurricane warning is issued when these conditions are likely
in your area within 24 hours.
4 of 4
BENZING & ROSS, LLC
COMMUNITY EMERGENCY PLANNING & RESPONSE
How much energy does a hurricane release?
Hurricanes can be thought of, to a first approximation, as a heat
engine; obtaining its heat input from the warm, humid air over the
tropical ocean, and releasing this heat through the condensation of
water vapor into water droplets in deep thunderstorms of the eyewall
and rainbands, then giving off a cold exhaust in the upper levels of
the troposphere (~12 km/8 mi up). One can look at the energetics of
a hurricane in two ways:
1. the total amount of energy released by the condensation of
water droplets or ...
2. the amount of kinetic energy generated to maintain the strong
swirling winds of the hurricane.
It turns out that the vast majority of the heat released in the
condensation process is used to cause rising motions in the
thunderstorms and only a small portion drives the storm's horizontal
winds.
Method 1
An average hurricane produces 1.5 cm/day of rain inside a circle of
radius 665 km . Converting this to a volume of rain gives 2.1 x 1016
cm3/day. A cubic cm of rain weighs 1 gm. Using the latent heat of
condensation, this amount of rain produced gives
5.2 x 1019 Joules/day or 6.0 x 1014 Watts.
This is equivalent to 200 times the world-wide electrical generating
capacity - an incredible amount of energy produced!
Method 2
For a mature hurricane, the amount of kinetic energy generated is
equal to that being dissipated due to friction. One could either
integrate a typical wind profile over a range of radii from the
hurricane's center to the outer radius encompassing the hurricane.
Doing the latter and using 90 mph winds on a scale of radius 40
n.mi., one gets a wind dissipation rate (wind generation rate) of:
1.3 x 1017 Joules/day or 1.5 x 1012Watts.
This is equivalent to about half the world-wide electrical Generating
capacity.
5 of 5
BENZING & ROSS, LLC
COMMUNITY EMERGENCY PLANNING & RESPONSE
CONVERSION FACTORS
For winds:
1 mile per hour = 0.869 international nautical mile per hour(knot)
1 mile per hour = 1.609 kilometers per hour
1 mile per hour = 0.4470 meter per second
1 knot = 1.852 kilometers per hour
1 knot = 0.5144 meter per second
1 meter per second = 3.6 kilometers per hour
For pressures:
1 inch of mercury = 25.4 mm of mercury = 33.86 millibars
= 33.86 hectoPascals
For distances:
1 foot = 0.3048 meter
1 international nautical mile = 1.1508 statute miles
= 1.852 kilometers = .99933 U.S nautical mile (obsolete)
1° latitude = 69.047 statute miles = 60 nautical miles
= 111.12 kilometers
For longitude the conversion is the same as latitude except the
value is multiplied by the cosine of the latitude.
6 of 6
BENZING & ROSS, LLC
COMMUNITY EMERGENCY PLANNING & RESPONSE
Saffir-Simpson Hurricane Scale
Sustained
Scale
Winds
Number
(Category) (MPH)
Damage
Storm
Surge
1
74-95
Minimal: Unanchored mobile homes,
vegetation and signs.
4-5 feet
2
96-110
Moderate: All mobile homes, roofs,
small crafts, flooding.
6-8 feet
3
111-130
Extensive: Small buildings, low-lying
roads cut off.
9-12 feet
4
131-155
Extreme: Roofs destroyed, trees
down, roads cut off, mobile homes
destroyed. Beach homes flooded.
13-18 feet
5
More than Catastrophic: Most buildings
155
destroyed. Vegetation destroyed.
Major roads cut off. Homes flooded.
7 of 7
Greater
than 18
feet