2004-09-03 / Columnists

Drawing On Science


The period between late August and early September is known as Hurricane Season. Hurricanes are the most destructive meteorological forces on earth. In the Far East these storms are called typhoons or cyclones .

The destructive forces of a hurricane are due to three things: strong winds, rain, and storm surges. Winds create a dome of water in the ocean, pushing it in the direction that the hurricane is going. This dome of water can result in a storm surge when it reaches land. The winds also carry debris and rip structures apart when the hurricane reaches land. Excessive rains create flooding with or without the aid of a storm surge. An additional threat comes from the possible formation of tornadoes.

Hurricanes form when waters off the coast of West Africa begin to warm during the summer months. It is the heat from these waters that fuel a hurricane. The combination of warm water and atmospheric movement results in thunderstorms. Winds push the storms over the ocean toward the Cape Verde Islands. If the waters in this region reach 80 o F or more, warm air rises and cools creating more storm clouds and the storms intensify becoming a tropical depression . Once the tropical depression has winds of 40-65 mph, it becomes a tropical storm. At this time the storm is more organized and starts to take on a circular, spiral shape. When the winds reach 75 mph a spiraling movement develops around the central core, which becomes the eye of the hurricane. In the northern hemisphere this spiraling movement is counterclockwise. In the southern hemisphere it is clockwise. This is the most recognizable feature of a hurricane and it is the region where the lowest surface pressures (the weight of the air above the surface) are found. Surrounding the eye is a region of intense winds and heavy rainfall. This is the eye wall . The eye itself is a relatively calm, clear, sunny area. At the same time large bands of rain clouds spiral out from the eye wall and are known as rain bands (diagram A).

When water vapor condenses into liquid it releases heat. The heat warms the surrounding atmosphere, which causes air to rise and leave the center of the storm. This is called the and the surface pressure decreases. As this pressure decreases, a larger pressure gradient (the change in pressure over a given area) occurs. The increased pressure draws more air to spiral in toward the center of the hurricane. This causes more warm, moist surface air to rise again. The air, once more, cools and condenses into clouds. While this is occurring, the air releases more heat and the pressure gradient continue to increase drawing more air down into the center. This cycle of cool and warm air spiraling into and out of the hurricane’s center continues to repeat itself giving strength to the hurricane. The three things that will cause the hurricane to weaken are cool water, land, or high wind shear (=the amount of change in wind speed or direction with increasing altitude).

Hurricanes are unpredictable as to how much force they will attain and the path that they will follow. They are the most powerful and destructive weather systems on earth simply because of the large area that they can cover.

A hurricane’s forward speed averages 15-20 mph and some have been clocked at 60 mph. If a hurricane stalls, its rainfall can be devastating. A small hurricane’s winds can reach out to about 25 miles from the center. A large hurricane’s winds can extend out to more than 150 miles. Tropical storm-force winds can cover an area of 350 miles! But it is the eye of a hurricane that determines how large and how strong the hurricane is. A small eye means that the hurricane will cover less area, but wind speed is very high. A large eye means that the hurricane will cover a larger area, but its wind speed is not as high.

In general the right side of a hurricane (determined by the direction in which it is moving) is the most destructive (diagram B). This is because of the combination of the hurricane’s wind speed and the speed of the larger steering winds (atmospheric winds). The combined wind speed increases the chance of a storm surge. The wind speed on the hurricane’s left is less. This is because the wind is moving against the steering wind speed. Now if this is not enough keep in mind that the most violent winds are at the backside of the hurricane’s eye. As the eye crosses the coast the storm’s forward speed is now combined with the high speed of winds being sucked in directly from shore. This gives the area being overrun by a hurricane a double blow. First the forward part of the hurricane, followed by the calm of the eye, and finally the more intense backside of the hurricane.

The strength of a hurricane is determined by the Saffir-Simpson Hurricane Scale: Category 1= winds 74-95 mph, storm surge generally 4-5 ft above normal; Category 2= winds 96-110 mph, storm surge generally 6-8 ft above normal; Category 3= winds 111-130 mph, storm surge generally 9-12 ft above normal; Category 4= winds 131-155 mph, storm surge generally 13-18 ft above normal; Category 5= winds greater than 155 mph, storm surge generally 18 ft above normal.

It is wise to heed the warnings of those who know hurricanes best. If orders for the evacuation of your coastal community are given, pack up and leave!

Questions/comments? E-mail Steve:Drawingonscience@aol.

com.

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