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.

There are three distinct structural regions of a hurricane:

Hurricane Mitch (1998) - CAT 5

• Eye - a clear region in the center of the hurricane that features sinking air and relatively calm weather. Seas underneath the eye are extremely dangerous however. The lowest pressure will be found in the center of the eye.
• Eyewall - as warm, humid air rushes in toward the eye, it spirals around and up to form large cumulonimbus clouds (thunderstorms). This ring of very intense thunderstorms surrounding the eye is where the highest winds and strongest storm surge will be found.
• Spiral Bands - bands of thunderstorms that spiral away from the eyewall for hundreds of miles. The spiral bands will be the first thunderstorms that make landfall and can cause heavy rains, gusty winds, and in some cases, tornadoes.

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

• Storm Surge - a dome of water 40 to 60 miles long that moves onto the shoreline near the landfall point of the eye. A cubic yard of sea water weighs approximately 1,700 pounds. As this water is constantly slamming into shoreline structures, even well-built structures quickly get demolished. As the waters move inland, more debris floats along with it causing further damage. Storm surge is responsible for nearly 90% of all hurricane-related deaths and injuries. A detailed analysis of the extent of storm surge inundation of the south shore of Long Island follows in the next section.
• Wind Damage - the force of wind can quickly decimate the tree population, down power lines and utility poles, knock over signs, and may be strong enough to destroy some homes and buildings. Flying debris can also cause damage, and in cases where people are caught outdoors, injuries and death. When hurricanes first make landfall, it is common for tornadoes to form which can cause severe localized wind damage. In most cases, however, wind is a secondary cause of damage. Storm surge is normally the primary cause.
• Rainfall & Flooding - the torrential rains that normally accompany a hurricane can cause serious flooding. Whereas the storm surge and high winds are concentrated near the eye, the rain may extend for hundreds of miles and may last for several days, affecting areas well after the hurricane has diminished.

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Scott A. Mandia, Professor - Physical Sciences T-206 Smithtown Sciences Bldg. 451-4104 mandias@sunysuffolk.edu <-- PREFERRED CONTACT METHOD http://www2.sunysuffolk.edu/mandias/ BOBBY v.3.2 approved site