1938 Hurricane

Introduction to Hurricanes

Hurricanes start out as a region of thunderstorms called easterly waves that drift across the tropical oceans. If the area of thunderstorms shows decreasing pressure, rotation (circulation), and wind speeds greater than 23 mph, it will be called a tropical depression and will be assigned a number. Perhaps 100 depressions may form in a normal season around the world, but few of these depressions ever become hurricanes. If conditions are favorable, the sustained wind speeds around the center of low pressure may reach 40 mph. At this point, the depression is called a tropical storm and will be assigned a name by the National Hurricane Center. If conditions are extremely favorable for development (sea-surface temperatures > 80 oF, low pressure at sea-level, and high pressure aloft), the maximum sustained winds may reach 74 mph. At this point, the tropical storm will be called a hurricane.

Hurricanes are classified according to the Saffir-Simpson scale shown below. One should note that the power of wind increases as the cube of the wind speed so that a doubling of wind speed causes an 8-fold increase in wind power! Therefore, a category 5 hurricane's wind is at least 8 times more powerful than a minimal category 1 hurricane.

Saffir-Simpson Scale for Hurricane Classification
Strength Wind Speed
(kt)
Wind Speed
(mph)
Pressure
(millibars)
Pressure
(inches Hg)
Storm Surge
(ft.)
Category 1 65-82 kt 74-95 mph >980 mb >28.94 in.
4-5 ft.
Category 2 83-95 kt 96-110 mph 965-979 mb 28.50-28.91 in.
6-8 ft.
Category 3 96-113 kt 111-130 mph 945-964 mb 27.91-28.47 in.
9-12 ft.
Category 4 114-135 kt 131-155 mph 920-944 mb 27.17-27.88 in.
13-18 ft.
Category 5 >135 kt >155 mph <919 mb <27.16 in.
>18 ft.
Tropical Cyclone Classification
Tropical Depression 20-34 kt or 23-39 mph
Tropical Storm 35-64 kt or 40-73 mph
Hurricane 65+ kt or 74+ mph

There are three distinct structural regions of a hurricane:

Damage caused by a hurricane can be divided into three categories:

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Scott A. Mandia, Professor - Physical Sciences
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http://www2.sunysuffolk.edu/mandias/

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