Under pleasant blue skies, a road sign in southern Florida is a reminder of the powerful storms that ravage the east coast of the US almost every year.

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WHAT are hurricanes?

Tropical cyclones are strong low pressure cells that spend most of their lives over the oceans. In the Atlantic Ocean, they are called hurricanes. In the Pacific Ocean, they are usually referred to as typhoons. Officially, a storm is classified as a hurricane when its sustained winds reach 119 kph (74 mph) or greater. Beyond this wind speed, hurricanes are classified on the Saffir-Simpson scale. Hurricanes can span hundreds of kilometers and cause damage well beyond their borders. Although typically slow moving at 25-35 kph (15-22 mph), they can occasionally travel at over 100 kph (62 mph).

In a hurricane, building doors and windows are frequently broken by debris picked up in the hurricane's strong and sustained winds. These winds can also rip roofs from buildings, topple trees, and damage power and communication lines. In some cases, hurricanes can produce tornado-like vortices (called "mini-swirls") which can completely destroy entire buildings. As dangerous as these winds are, they are usually not responsible for most of the damage in a hurricane. Coastal flooding is the major threat in hurricanes. This flooding is most often the combination of storm surges and torrential rain. Storm surges are rises in ocean levels produced by the effects of high wind and low atmospheric pressure. Storm surges can come ashore rapidly with heights over 20 feet. Besides causing flooding, storm surges can also increase coastal erosion, potentially resulting in slope failures. Hurricanes can even start fires by damaging power lines. Contamination of drinking water and disruption of utility services (such as electricity, communications, and sewer) are common occurrences during a hurricane. Long after a hurricane, stagnant floodwaters can produce water-borne and insect-borne diseases.

WHERE do hurricanes occur?

Hurricanes are born over the tropical oceans. They require vast amounts of warm, moist air to survive. Hurricanes lose strength over land or cool ocean water and are therefore confined by latitude and the location of land masses. The Northern Hemisphere usually has about twice the number of tropical cyclones as the Southern Hemisphere (averaging 55 storms per year vs. 25 storms per year).

Atlantic Ocean: On average, less than two hurricanes per year reach the eastern seaboard of North America from the western coast of Africa. However, several more usually come close enough to the coast to cause some damage. In the western Atlantic Ocean, an average of nine hurricanes occur each year. During El Nino events, fewer hurricanes tend to form in the Atlantic Ocean, while more hurricanes tend to form in this area during La Nina events.

Pacific Ocean: Approximately half of all northern hemisphere hurricanes occur in the western region of the North Pacific Ocean. These typhoons impact the countries of Japan, Taiwan, China, South Korea, and North Korea.

Indian Ocean: The southeast Indian Ocean experiences the greatest number of tropical cyclones in the Southern Hemisphere with an average of ten per year. Approximately eight tropical cyclones occur in the southwest Indian Ocean each year. About six hurricanes are reported in the northern Indian Ocean annually.

WHEN do hurricanes occur?

Hurricanes originate over warm water (usually 26°C) or warmer) and they need warm, moist air to sustain their energy. Such conditions usually occur during the Northern Hemisphere's summer and fall seasons. Therefore, the hurricane season for North America usually begins in early June and usually ends in late November. However, during El Nino and La Nina years, this season may be altered slightly, both in length and timing. During the 2005 hurricane season, hurricanes were formed as late as January, 2006.

Northern Hemisphere: Approximately 70% of of all hurricanes in the Northern Hemisphere occur between the months July and October. In the western North Pacific Ocean, hurricanes are most common during this time. In the eastern North Pacific Ocean, the usual season of occurrence is confined more to the months of August and September. The northern Indian Ocean usually has a two-part hurricane season: May through June and October through November.

Southern Hemisphere: In the oceans of the Southern Hemisphere, the hurricane season tends to be considerably shorter. Approximately 70% of all hurricanes occur between January and March in both the southern Indian Ocean and South Pacific Ocean.

HOW do we cope with hurricanes?

Hurricanes are capable of mass destruction and have been one of North America's most costly disasters. The final cost of Hurricane Katrina may approach half of a trillion dollars, making it the costliest disaster in history. Fortunately, hurricanes are relatively slow moving, which often allows for adequate warning and evacuation of people living in areas where hurricanes threaten. However, hurricanes are cover a large area and can sometimes unexpectedly change their direction and intensity, catching people off guard. Also, one part of a hurricane may produce significantly faster winds than other parts. Accordingly, the National Hurricane Center issues hurricane warnings and watches for communities along extensive coastline areas to alert residents of the threat.

Buildings can be constructed to be hurricane-resistant by tying down rafters and including strong metal shutters to cover windows and doors in the event of a hurricane. Individuals can prepare for hurricanes by stocking appropriate materials such as food, water, batteries, and other essential items, or evacuating if a hurricane is expected to come close to their community.

WHO has more information on hurricanes?

• NOAA National Hurricane Center
  www.nhc.noaa.gov