Storm Structure
The process by which a disturbance forms and subsequently strengthens into a hurricane depends on at least three conditions. Warm waters and moisture are mentioned above. The third condition is a wind pattern near the ocean surface that spirals air inward. Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this structure can remain intact and allow for additional strengthening.
The center, or eye, of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall. At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.
Hurricane
A hurricane is a violent storm originating over tropical waters with sustained (constant) winds of over 74 miles per hour. The winds, as in tropical depressions and storms, blow in a counterclockwise direction around the center. The diameter of these storms may range anywhere from 100 to 1,000 miles.
To make the predicted hazards of an approaching hurricane easier to understand, hurricane forecasters have developed a disaster potential scale, which assigns storms into five categories. This scale is called the Saffir-Simpson Hurricane Scale.
Eye - The center portion of the storm around which winds and rain rotate. Winds are calm and skies are clear or partly cloudy when the eye of a storm passes.
Storm Surge - A large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall. The stronger the hurricane and the shallower the offshore water, the higher the surge will be along the coast. Storm surge is the greatest threat to life and property.
Storm Tide - The combination of the storm surge and the normal astronomical tide.
Advisory - Advisory messages are issued by the National Hurricane Center, concerning all tropical depressions, tropical storms, and hurricanes. An advisory gives details as to where the storm is located, its intensity, its direction and its speed of movement. Precautionary measures are given for ships and coastal residents near the storm center. Advisories are broadcast over most radio and television stations.
Hurricane Watch - A hurricane watch is issued for coastal waters and immediate adjacent land areas when hurricane conditions are expected for the area within 36 hours. Hurricane conditions are sustained winds of 74 miles per hour or greater (64 knots...) associated with a tropical cyclone.
Hurricane Warning - A hurricane warning is issued when hurricane conditions are expected for coastal waters and immediate adjacent land areas within 24 hours.
Tropical Storms
Tropical Storm Watch - An announcement is made for specific coastal areas where tropical storm conditions pose a possible threat within 36 hours.
Tropical Storm Warning - A warning that tropical storm conditions, including possible sustained winds within the range of 39 to 73 miles per hour.
Tropical Depression - A tropical depression is a closed circulation of low atmospheric pressure, originating over tropical waters, that blow counterclockwise around a center at speeds less than 39 miles per hour.
Tropical Storm - A tropical storm is a closed circulation of low atmospheric pressure, originating over tropical waters, that blow counterclockwise around a center at speeds ranging from 39 to 73 miles per hour.
Non-Tropical Storms
Gale Warning - A gale warning is issued when sustained winds of 39 to 53 miles per hour (34 to 47 knots) associated with a non-tropical storm are expected over the coastal waters and immediate adjacent land areas.
Storm Warning - A storm warning is issued when sustained winds of 55 miles per hour (48 knots) associated with a non-tropical storm are expected over coastal waters and immediate adjacent land areas.
Marine
Special Marine Warning - Special marine warnings are issued when short-lived frequent wind gusts of 34 knots (39 miles per hour) or greater are occurring or expected to occur over water. These winds are most often associated with thunderstorms. Special marine warnings are also issued when waterspouts are observed or expected to occur over water.
Small Craft Advisory - Small craft advisories are issued when sustained winds of 21 to 33 knots (24 to 38 miles per hour) or seas of seven feet or greater or are occurring or are expected to occur over water.
Small Craft Exercise Caution - This is issued when sustained winds of 15 to 20 knots (17 to 23 miles per hour) or seas of six feet are occurring or are expected to occur over the water.
Marine Weather Statements - Marine weather statements are issued to advise boaters of significant weather phenomena that pose a threat to small boats but do not reach the criteria of the other warnings or advisories.