Hurricane: Overview

Hurricanes and typhoons are different names for a strong "tropical cyclone." A tropical cyclone is a low-pressure system occuring over tropical or sub-tropical waters with definite cyclonic surface wind circulation of over 74 mph. Depending on their geographical location, tropical cyclones are called:

Hurricanes
in the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E
Typhoon
in the Northwest Pacific Ocean west of the dateline
Severe Tropical Cyclone
in the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E

Hurricanes are maintained by positive feedback loops in which condensations of moisture in rising air flow out through the eye of the storm and drop back toward the ground and form winds in the process. Because condensation is such a vital part of the process, hurricanes must remain over warm water, continuing to receive atmospheric moisture. Hence, hurricanes rapidly lose force once they make landfall. Tropical cyclone activity generally peaks in the latter part of summer when water temperatures are warmest. All in all, about 80 tropical cyclones form each year.

Tropical cyclones can form in any of the several basins: the North Atlantic Basin, Western North Pacific Ocean, Eastern North Pacific Ocean, South Western Pacific Ocean, Northern Indian Ocean,  Southeastern Indian Ocean, Southwestern Indian Ocean. In order for such hurricanes to occur, there are five conditions that need to be met:

  1. Sea surface temperatures above 26.5 degrees Celsius (79.7 degrees Fahrenheit)
  2. Temperature in the atmosphere decreases quickly with height, and the mid-troposphere must be relatively moist.
  3. A pre-existing weather disturbance such as tropical waves
  4. A distance of 10 degrees or more from the equator, so that the Coriolis effect is strong enough to cause the cyclone to rotate
  5. Low vertical wind shear

Hurricanes are ranked by their maximum winds using the Saffir-Simpson Hurricane Scale. In the ranking of 1 to 5, a Category 5 hurricane is the strongest. However, rankings do not predict the power to deal damage. In fact, lower-category hurricanes often times inflict greater damage than higher-category ones.