Hurricane FormationDue to the Coriolis effect, the regions between 5° and 20° north and south of the equator are the belts where hurricanes can form (there is not enough rotary motion between 5° north and south. The term cyclone is used in the Bay of Bengal and Arabian Sea and the term typhoon is used in the Pacific Ocean north of the equator and west of the International Dateline.
The birth of a hurricane starts as a low pressure zone and builds into a tropical wave of low pressure. In addition to a disturbance in the tropical ocean water, the storms that become hurricanes also require warm ocean waters (above 80°F or 27°C down to 150 feet or 50 meters below sea level) and light upper level winds.
Growth and Development of Tropical Storms and HurricanesA tropical wave grows in intensity and then may grow to become an organized area of showers and thunderstorms known as a tropical disturbance. This disturbance becomes an organized area of tropical low pressure that is called a tropical depression based on cyclonic winds (counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). A tropical depression's wind speed must be at or below 38 miles per hour (mph) or 62 km/hr when averaged out over one minute. These winds are measured at 33 feet (10 meters) above the surface.
Once average winds reach 39 mph or 63 km/hr then the cyclonic system becomes a tropical storm and receives a name while tropical depressions are numbered (i.e. Tropical Depression 4 became Tropical Storm Chantal in the 2001 season.) Tropical storm names are preselected and issued alphabetically for each storm.
There are approximately 80-100 tropical storms annually and about half of these storms become full-fledged hurricanes. It is at 74 mph or 119 km/hr that a tropical storm becomes a hurricane. Hurricanes can be from 60 to almost 1000 miles wide. They vary widely in intensity; their strength is measured on the Saffir-Simpson scale from a weak category 1 storm to a catastrophic category 5 storm. There were only two category 5 hurricanes with winds over 156 mph and a pressure of less than 920 mb (the world's lowest pressures ever recorded were caused by hurricanes) that struck the United States in the 20th century. The two were a 1935 hurricane that struck the Florida Keys and Hurricane Camille in 1969. Only 14 category 4 storms hit the U.S. and these included the nation's deadliest hurricane - the 1900 Galveston, Texas hurricane and Hurricane Andrew which hit Florida and Louisiana in 1992.
Hurricane damage results from three primary causes:
1) Storm Surge. Approximately 90% of all hurricane deaths can be attributed to the storm surge, the dome of water created by the low pressure center of a hurricane. This storm surge quickly floods low-lying coastal areas with anywhere from 3 feet (one meter) for a category one storm to over 19 feet (6 meters) of storm surge for a category five storm. Hundreds of thousands of deaths in countries such as Bangladesh have been caused by the storm surge of cyclones.
2) Wind Damage. The strong, at least 74 mph or 119 km/hr, winds of a hurricane can cause widespread destruction far inland of coastal areas, destroying homes, buildings, and infrastructure.
3) Freshwater Flooding. Hurricanes are huge tropical storms and dump many inches of rain over a widespread area in a short period of time. This water can engorge rivers and streams, causing hurricane-induced flooding.
Unfortunately, polls find that about half of Americans living in coastal areas are unprepared for a hurricane disaster. Anyone living along the Atlantic Coast, the Gulf Coast and the Caribbean should be prepared for hurricanes during hurricane season.
Fortunately, hurricanes ultimately diminish, reverting to tropical storm strength and then into a tropical depression when they move over cooler ocean water, move over land, or reach a position where the upper level winds are too strong and are thus unfavorable.