To put it simply, a hurricane is an extremely low-pressure compound thunderstorm, or storm system, that has been upgraded from a "tropical storm" due to its intensity. A tropical storm has sustained winds of less than 73 mph (about 118 kph), but anything above that causes the storm to be upgraded to a hurricane. Hurricanes may contain thunder, lightning, rain and wind gusts well over 100-200 mph (161-322 kph).
Hurricanes in the Northern Hemisphere rotate in a counter-clockwise direction, while storms from the Southern Hemisphere move clockwise. The latter type of storm is usually referred to as a typhoon.
Hurricanes grow in intensity due to the affects of air and ocean conditions, such as humidity and water temperature. Ocean water must be over 81° Fahrenheit (26.5° Celsius) to energize a hurricane. It is the heat from the water, combined with the humidity in the air, that powers the storm. Once a hurricane hits land, or cooler water temperatures, it quickly loses strength.
When just the right conditions are met, a storm can develop into a hurricanes within a matter of hours, although at times it can take up to several days. Some storms seem to fizzle out, but they may begin to build again as they move into warmer waters. Consequently, such storms must be watched carefully. A hurricane can cause catastrophic damage. Watches and warnings are put into effect as soon as feasible in order to warn people of impending danger and to give them a chance to evacuate or take precautions.
The strength of a hurricane is categorized in accordance with the Saffir-Simpson scale. This scale determines whether the storm should be labeled a category 1, 2, 3, 4, or 5. A category 1 is the lowest risk storm, while a category 5 is less common, but catastrophic if it makes landfall.
The National Hurricane Center observes weather conditions common with hurricane activity. Once those at the Center decide that such activity poses a possible danger for islands and coasts, with dangerous conditions that could affect a certain area within 36 hours, a hurricane watch or "advisory" is issued. Such advisories inform people and businesses of the hurricane's strength, location and path, giving them the earliest possible chance to take precautions or evacuate the area.
A hurricane warning means that the area under the warning can expect to see hurricane activity within the next 24 hours. A warning may be in effect even where winds are less than the standard 74 mph, if conditions present the risk of high waves and dangerous water levels. Once a warning has been issued, precautionary action should be taken calmly and immediately. A hurricane warning should be considered, and dealt with, as an emergency.