17th April 2006
Hurricane
Source: Excerpt from The Book " Weather "
Distribution:
Between 5
degrees and 30 degrees North and South.Height: Around 60,000 feet. Cause: Intense convection over warm tropical oceans, combined with instability. Associated Weather: Destructive winds and rain. Hazard Warning: Extreme threat to life and property. Little else in nature has the destructive force of a fully fledged hurricane. These massively powerful systems can produce sustained winds of 150 miles per hour (250 kph) with gusts of up to 190 miles per hour (300 kph), as well as intense bursts of rain and ocean surges that cause extensive flooding. While tornadoes can produce even stronger winds, they rarely last for more than a few hours. A hurricane, on the other hand, can last for weeks and cover thousands of miles.
A mature hurricane consists of bands of thunderclouds spiraling around the eye -- a clear, almost calm area at the center of the storm. The whole storm system may contain hundreds of thunderstorms and measure up to 600 miles (970 km) in diameter. To qualify as a hurricane, a storm must produce winds of over 74 miles per hour (119 kph).
In the Northern Hemisphere, rotating systems with lower wind speeds are known as tropical storms or tropical depressions. In the western Pacific and China Sea area, hurricanes are known as typhoons, from the Cantonese tai_fung, meaning great wind. In Australasia and countries around the Indian Ocean, the same storm systems are known as tropical cyclones.
The clusters of storms that produce hurricanes occur only where sea temperatures are at least 80° F (27° C). This means that they usually originate in the tropics. To develop its distinctive rotation, the system must be at least 5 degrees from the equator, because this is where the Coriolis effect begins to have an influence. Once spinning, a storm system tends to move farther away from the equator, although it is unlikely to continue beyond 30 degrees north or south. If a hurricane returns toward the equator, it usually begins to weaken. It is impossible for a hurricane to cross the equator, because there the Coriolis effect has no impact and the hurricane will lose all its rotational energy and decay into a cluster of thunderstorms once again.
Hurricane Folklore
Destructive hurricanes have been recorded many times throughout history, with accounts of cities being destroyed and shipping fleets sunk by furious winds and mountainous seas. The naval fleets of the Mongol emperor, Kublai Khan, were scattered by typhoons in 1274 and again in 1281, while preparing to attack Japan. The Japanese, believing that these storms had been sent to protect their country from invasion, referred to them as divine winds, or kami-kaze.
Until recently, there was no sure way of knowing if a hurricane was approaching the coast, but often the sea provided some clues. A hurricane at sea produces a swell that spreads from the center of the system and often runs well ahead of the storm. Thus, if a large swell is observed, particularly when there is little local wind, this may be a sign of a hurricane. From a high vantage point, it should be possible to observe the direction from which the swell is approaching, and from this, deduce the direction in which the hurricane lies. The farther the waves are from the storm, the greater the distance between them; so, if swell waves are breaking in increasingly rapid succession as the day progresses, a hurricane may be approaching.
Modern Surveillance
The development of satellite photography has allowed people to fully appreciate the spectacle and majesty of these revolving spirals of cloud and permitted scientists to monitor their movements closely.
Satellite images now form the basis for hurricane forecasting around the world and,as meteorologists have become more expert at interpreting them, hurricane forecasting has continued to improve. During the local hurricane season, meteorologists receive satellite photographs hourly. They look for large clusters of thunderstorm clouds over tropical oceans, which may be embryonic hurricanes. Once such a cluster is identified, scientists monitor sequences of satellite images for any sign of rotation.
If the satellite imagery reveals a rotating system moving into higher latitudes, the meteorologist looks for evidence of eye formation. Once this has been identified, and the surface wind speed has reached 74 miles per hour (119 kph), the weather service in question will then declare it to be a hurricane, and it will be officially named from the list provided by the World Meteorological Organization. A continuous watch is then kept on the hurricane, with warnings disseminated to shipping, aircraft, and the general public as long as the storm persists.
If a hurricane moves to within about 150 miles (240km) of the coastline, it is then within radar range, and can be accurately tracked. However, even at this stage, there is an element of uncertainty as hurricanes behave unpredictably. It may continue in a straight line, or it may stall or veer. The reasons for this erratic movement are the subject of continuing research.
In order to study hurricanes, research scientists undertake flights into the eye of the storm (as shown in the picture on the right). The towering ramparts of cumulonimbus clouds that surround the eye may measure upto 60,000 feet high.
The hurricane poses the greatest danger to human life once it reaches the coastline. Loss of life can be minimized by evacuating an area, but destruction of property is unavoidable. The surge of sea water associated with a hurricane can inundate large areas of coastal land, while high winds and further flooding from rain may cause additional damage.
As the hurricane crosses the coastline and moves away from the sea -- its source of energy and moisture -- it begins to dissipate rapidly, although rains may continue for a few days.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
