Hurricanes

 

Hurricanes form in tropical regions where there are warm water (80 degrees Fahrenheit or better), moist air and converging winds. Three events must happen for hurricanes to form:

A continuing evaporation-condensation cycle of warm, humid ocean air
Patterns of wind with converging winds at the surface and strong, simultaneous winds at higher altitudes
A difference in air pressure (i.e., a pressure gradient) between the surface and high altitude
The whole process of hurricane formation is not entirely understood and it gets technical, but here is an overview in 3 steps (if you would rather not get technical, click here):
  1. Warm, moist air from the ocean surface begins to rise rapidly. As this warm air rises, the water vapor in it condenses to form droplets of storm clouds and rain. The condensation releases heat called latent heat of condensation. This latent heat warms the cool air aloft, thereby causing it to rise too. This rising air is replaced by more warm, humid air from the ocean below. This cycle continues drawing more warm moist air into the developing storm and serves as an engine to move heat from the surface to the atmosphere. This exchange of heat from the surface by rising humid air creates wind that circulates around a center. The circulation is similar to the circulation process you see in water going down the drain.
  2. "Converging winds" are winds moving in different directions that run into each other at a certain spot. Converging winds at the surface collide and push more warm, moist air upward. This rising air reinforces the rising air from step 1, so the circulation and the wind speeds increase. Meantime, strong winds blowing at the same speeds at all levels of the atmosphere at higher altitudes (up to 30,000 feet (9,000 meters)) help to remove the rising hot air away from the storm's center. This helps to maintain continual movement of warm air from the surface and to keep the storm organized. If the high altitude winds do not blow at the same speeds at all levels (i.e., wind shears are present), then the storm will lose organization and weaken.
  3. High pressure air in the upper atmosphere above the storm's center (greater than 30,000 ft (9,000 m)) also helps to remove the heat from the rising air and drives the hurricane's growth. As high pressure air moves toward the low pressure in the center of the storm, the winds increase speed. See the links section for more details.
By drawing humid air from the surface, increasing its wind speeds and increasing the pressure gradient, the storm develops in stages:
Tropical depression: Swirling clouds and rain with wind speeds less than 38 mph (33 knots).
Tropical storm: Wind speeds of 39 to 73 mph (34 to 63 knots).
Hurricane: Wind speeds greater than 74 mph (64 knots).

The proceeding is from http://www.howstuffworks.com/hurricane1.htm

 

Hurricane safety

Hurricane Warning: A warning that sustained winds 64 kt (74 mph or 119 kph) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

Hurricane Watch: An announcement of specific coastal areas that a hurricane or an incipient hurricane condition poses a possible threat, generally within 36 hours.

Tropical Storm Warning: A warning for tropical storm conditions including sustained winds within the range of 34 to 63 kt (39 to 73 mph or 63 to 118 kph) that are expected in a specified coastal area within 24 hours or less.

Tropical Storm Watch: An announcement that a tropical storm poses or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A tropical storm watch should normally not be issued if the system is forecast to attain hurricane strength.

The links below will help you learn what to do when a watch or warning is issued. Even more important, they will help you prepare before the hurricane season begins. Those who wait until a watch or warning are issued to obtain supplies have generally waited too long.

Tips to follow when hurricane watches, warnings are issued.
A USA TODAY Online graphic explains some basic ways to prepare your home for the hurricane season. Another file gives additional tips on how to prepare for a hurricane.
More information on hurricane preparedness is available on the Federal Emergency Management Agency's Hurricane Page.
An article from the Federal Emergency Management Agency's hurricane page explains why tropical storms should not be underestimated.
Once a hurricane moves on and the winds die down, the danger isn't over. USA TODAY Online text gives advice on how to stay safe after a storm.
The Metro/Dade (Miami) Building Code Compliance Office has information on preparing for hurricanes along with other useful tips for what to do after a storm hits.
Improved warnings, new shelters save thousands in Bangladesh

from http://www.usatoday.com/weather/huricane/whsafety.htm