Tropical Cyclone FAQ

What is the difference between a tropical disturbance, tropical depression, tropical storm, and a hurricane?

• A tropical disturbance is a discrete system of organized showers and thunderstorms that originates in the tropics and maintains its identity for 24 hours or more.
• A tropical depression is an organized system of clouds and thunderstorms with a defined counter-clockwise circulation with maximum sustained winds of 38 mph (33 knots) or less.
• A tropical storm is an organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 to 73 mph (34-63 knots).
• A hurricane is an intense tropical weather system with a well defined circulation and maximum sustained winds of 74 mph (64 knots) or higher.

When is hurricane season?

In the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico hurricane season extends from June 1st until November 30th. The peak hurricane threat exists from mid-August to late October. In other parts of the world, such as the western Pacific, hurricanes can occur year-round.

How is a hurricane formed?

Hurricanes usually originate as a cluster of showers and thunderstorms in tropical waters. The process by which a disturbance forms and subsequently strengthens into a hurricane depends on many conditions. However, there are three major factors in which must be met before hurricanem formation. Those factors include warm water and moisture, favorable low level winds, and light upper level winds.

Hurricanes main source of energy comes from heat. Developing hurricanes gather heat and energy through contact with warm ocean waters. Most of the time in the hurricane generation season, ocean water temperatures are warmer than 79 degrees.

Winds near the ocean surface need to rotate counter-clockwise (low pressure) in order for a cluster of storms to develop into a tropical system. This counter-clockwise flow directs winds and moisture inward to a central point. Once this moisture nears the central point, the center of a low pressure system, it is lifted into the atmosphere to form more showers and thunderstorms. As this process repeats itself, the more energy and moisture is available, and the area of showers and thunderstorms grows and intensifies leading to the formation of a tropical system.

The upper portion of the atmosphere is to a hurricane what a muffler is to a car. All of the exhaust from a car must go somewhere so it goes through a hollow tube (muffler) into the open air. If an object is placed in the muffler or it is clogged, the car will not run properly. The same situation can be applied to tropical systems. All the upward moving moisture and water vapor (exhaust) in the center of the low must go somewhere. Upper levels of the atmosphere act as the tropical systems "muffler". In an ideal situation, light upper level winds will allow this moisture to be "fanned out" to other parts of the storm. If there are strong winds aloft, they act as a lid to the "muffler" and the moisture has no place to go. Hence, the storm has no means to intensify and may even collapse if the upper level winds are strong enough.

What are the different parts of the hurricane?

The typical hurricane has 2 or 3 and sometimes more outer convective bands. These bands are comprised of cells resembling ordinary thunderstorms and can be up to 300 miles from the eye. The outer convective bands are generally 40 to 80 miles apart and come in advance of the main rain shield.

The rain shield is a solid or nearly solid area of rain that typically becomes heavier as one approaches the eye. The outer edge is well defined and its distance from the eye varies greatly from storm to storm. The wind, both sustained and peak gusts, keeps increasing as one moves through the rain shield toward the storm's eye.

Spiral convective rings or bands are regions of active showers and thunderstorms that encircle the centers of tropical cyclones. They are prevalent in the more intense hurricanes and curve cyclonically inward toward the center of the storm where they appear to merge to form the eye wall.

The eye wall is an organized band of thunderstorms that immediately surrounds the center or eye of a hurricane. It's generally around 15 miles wide and typically contains the fiercest winds and most intense rainfall.

The eye is a relatively calm center of the hurricane. The winds are light, and skies are partly cloudy or even clear. The average hurricane eye diameter is a little over 20 miles. In general, when the eye is shrinking in size, the hurricane is intensifying. During the daytime, the passage of the hurricane's eye over a community is usually accompanied by sunny skies and a rise in temperature. Birds may be flying with the eye for sanctuary. After the eye's passage, the violent wind howls in the opposite direction to what it was right before the eye moved over, and the heavy sheets of rain return.