A hurricane is a large rotating storm centred around an area of very low pressure, with strong winds blowing at an average speed in excess of 72 miles per hour. The whole storm system may be five to six miles high and 300 to 400 miles wide. It moves forward like an immense spinning top, at speeds of up to 30 m.p.h.
• A „hurricane“ is the most severe category of the meteorological phenomenon known as the „tropical cyclone.“
• Tropical cyclones are low pressure systems that have thunderstorm activity and rotate counterclockwise. A tropical cyclone that has winds of 38 mph (33 kt) or less is called a tropical depression. When the tropical cyclone’s winds reach 39-73 mph (34-63 kt), it is called a tropical storm. When the winds exceed 74 mph (64 kt), the storm is considered to be a hurricane.
• The Saffir-Simpson Hurricane Scale defines hurricane strength by categories. A Category 1 storm is the weakest hurricane (winds 74-95 mph or 64-82 kt); a Category 5 hurricane is the strongest (winds greater than 155 mph or 135 kt).
The category of the storm does not necessarily relate directly to the damage it will inflict. Lower category storms (and even tropical storms) can cause substantial damage depending on what other weather features they interact with, where they strike, and how slow they move.
A hurricane forms in the southern Atlantic Ocean, Caribbean Sea, Gulf of Mexico or in the eastern Pacific Ocean. Hurricanes need warm tropical oceans, moisture and light winds above them. If the right conditions last long enough, a hurricane can produce violent winds, incredible waves, torrential rains and floods.
Hurricanes rotate in a counterclockwise direction around an „eye.“ Hurricanes have winds at least 74 miles per hour. There are on average six Atlantic hurricanes each year; over a 3-year period, approximately five hurricanes strike the United States coastline from Texas to Maine.There is nothing like them in the atmosphere. Born in warm tropical waters, these spiraling masses require a complex combination of atmospheric processes to grow, mature, and then die. They are not the largest storm systems in our atmosphere or the most violent, but they combine these qualities as no other phenomenon does.
In the Atlantic Basin, they are called hurricanes, a term that echoes colonial Spanish and Caribbean Indian words for evil spirits and big winds. These awesome storms have been a deadly problem for residents and sailors ever since the early days of colonization. Today, hurricane damage costs billions.
C a u s e s o f h u r r i c a n e s
Hurricanes can develop when the sun’s rays heat tropical waters to at least 82 degrees Fahrenheit. This causes the air to grow warmer and rise. Water rises with it as vapor, and the heat causes the air to rise faster and faster. In about 12 hours, the heated air will begin to circle counterclockwise, forming stronger and stronger winds that whirl with increasing speed. Air that is motionless is calm. When it moves slowly, it is a light breeze. If it moves slightly faster, it becomes a fresh breeze, and even faster is a high wind. A wind becomes a hurricane when it reaches speeds of more than 74 miles (120 km) per hour. It is unknown what top hurricane speeds are, because instruments have always been destroyed by high winds or flying debris. Some estimate that hurricane winds do not surpass 250 miles (400 km) per hour, while others say they can reach 600 miles (960 km) per hour.
The moist, hot winds of a hurricane swirl upward around a calm column of low-pressure air known as the eye. Some hurricanes can develop more than one eye, but the largest is always the main eye and measures an average of 15 miles across. Inside the eye there are no storm clouds, almost no wind, and calm conditions. However, beneath this tranquil center is wild, stormy chaos.
The rising winds rotate in increasingly tight spirals around the eye, rising faster and faster. Cooler air is also sucked in, heated, and sent twisting up the eye. When the hot air rises high enough, it cools and forms water droplets that cluster together as clouds and drop rain. The rain is blown around by fierce winds.
Hurricanes were once thought to be shaped like doughnuts, circling around the eye. However, satellite images have shown them more to resemble pinwheels, with thicker clouds near the center and long, thin, trailing ends.
The atmospheric pressure is lowest closest to the eye, making the winds there spin faster. At the storm’s outer edges, they blow more slowly. A hurricane moves west or northwest at an average of ten miles an hour, with a wind radius as large as 100 miles. As it moves, it churns up the sea beneath the eye, sucking ocean water up and creating huge ocean waves. The raised water is carried along by the storm to the shore.
The Birth of a Tropical Cyclone
Tropical cyclones form over warm waters from pre-existing disturbances. These disturbances typically emerge every three or four days from the coast of Africa as „tropical waves“ that consist of areas of unsettled weather. Tropical cyclones can also form from the trailing ends of cold fronts and occasionally from upper-level lows.
The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on at least three conditions shown in the figure below
1. A pre-existing disturbance with thunderstorms
2. Warm (at least 80ºF) ocean temperatures to a depth of about
150 feet
3. Light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere (low wind shear) Heat and energy for the storm are gathered by the disturbance through contact with warm ocean waters. The winds near the ocean surface spiral into the disturbance’s low pressure area. The warm ocean waters add moisture and heat to the air which rises. As the moisture condenses into drops, more heat is released, contributing additional energy to power the storm. Bands of thunderstorms form, and the storm’s cloud tops rise higher into the atmosphere. If the winds at these high levels remain relatively light (little or no wind shear), the storm can remain intact and continue to strengthen.
The Eye
The hurricane’s center is a relatively calm, clear area usually 20-40 miles across. People in the midst of a hurricane are often amazed at how the incredibly fierce winds and rain can suddenly stop and the sky clear when the eye comes over them. Then, just as quickly, the winds and rain begin again, but this time from the opposite direction.
TheEyewal
The dense wall of thunderstorms surrounding the eye has the strongest winds within the storm. Changes in the structure of the eye and eyewall can cause changes in the wind speed, which is an indicator of the storm’s intensity. The eye can grow or shrink in size, and double (concentric) eyewalls can form.
The Spiral Rainbands
The storm’s outer rainbands (often with hurricane or tropical storm-force winds) can extend a few hundred miles from the center. Hurricane Andrew’s (1992) rainbands reached only 100 miles out from the eye, while those in Hurricane Gilbert (1988) stretched over 500 miles. These dense bands of thunderstorms, which spiral slowly counterclockwise, range in width from a few miles to tens of miles and are 50 to 300 miles long. Sometimes the bands and the eye are obscured by higher level clouds, making it difficult for forecasters to use satellite imagery to monitor the storm.