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Tropical terminology: What you need to know about hurricane season

The eye wall of Hurricane Ivan, shown from 230 miles above Earth, had sustained winds of 160 mph on Sept. 13, 2004.

Associated Press (2004)

The eye wall of Hurricane Ivan, shown from 230 miles above Earth, had sustained winds of 160 mph on Sept. 13, 2004.

What is a tropical storm?

It is a low-pressure, tropical system with maximum sustained winds near the center between 39 and 73 mph.

When does a tropical storm become a hurricane?

When maximum sustained winds near the center reach 74 mph.

What is a tropical depression?

It is a closed area of low pressure in the tropics with somewhat organized convection and sustained winds less than 39 mph.

What is the cone of uncertainty?

This is sometimes called the "cone of error." This shows the historical error at certain time periods in a tropical cyclone forecast. Average error in track forecasting over the past several years is at about 75 miles for 24 hours, meaning the "cone" will be 150 miles across. At the 120-hour forecast, the average error is 300 miles. It is important to realize that sometimes the actual forecast scenario may be more or less accurate than the historical error cone.

What is the consensus model?

With so many computer models to look at, groups of models get put together, and a "consensus" is the average of those models. This has become the newest and more accurate way to forecast hurricanes.

Is storm surge different from tidal surge?

Yes. A storm surge is the onshore rush of seawater pushed by high winds of an approaching or landfalling tropical storm or hurricane. Storm surge is secondarily affected by the low pressure of the storm. Tidal surge refers to the rise in water associated with high tide.

What is a Cape Verde hurricane?

It is a tropical cyclone that develops near the Cape Verde Islands just off the west coast of Africa and becomes a hurricane before reaching the Caribbean Sea. The peak of the Cape Verde season is August and September, and a typical year would have one to five hurricanes.

Who are the hurricane hunters?

Pilots and planes that fly into hurricanes to take measurements. In the Atlantic Hurricane Basin, reconnaissance is carried out by the Air Force Reserves' 53rd Weather Reconnaissance Squadron and the National Oceanic and Atmospheric Administration's Aircraft Operations Center. The 53rd is based at Keesler Air Force Base in Mississippi and uses WC-130 planes that have been modified to carry instruments that measure wind, pressure, temperature and dew point. The Oceanic and Atmospheric Administration is based at MacDill.

What is the difference between a watch and warning?

A watch is issued when a hurricane or tropical storm is possible within 36 hours. A warning is issued when those same conditions are expected within 24 hours.

Can a stationary storm weaken because it isn't moving?

It could weaken due to a process called upwelling, in which water rises from a lower depth to a higher depth. With respect to tropical weather, upwelling can occur if a storm is stationary or slowly moving, as the wind from the storm pushes the surface water away from the center. The lower-depth water is colder, and thus a stationary hurricane could actually weaken by bringing up that colder water.

How does a typhoon differ from a hurricane?

A typhoon and a hurricane are essentially the same except their location: A typhoon is a hurricane in the western Pacific Ocean.

What is the Bermuda High?

High pressure is a semiconstant feature in the Atlantic during summer and autumn. The high is centered near Bermuda and can extend across the entire Atlantic. High pressure in the Northern Hemisphere has clockwise wind circulation. The southern periphery of the high is a good indicator for the path of a tropical storm or hurricane.

What is a weakness, and how does it affect the path of a hurricane?

A weakness is the term given to the weakening side of an area of high pressure. Often a tropical storm or hurricane will follow the periphery of high pressure. If there is a weakness, the storm will tend to curve toward that weakness.

What is a millibar?

A millibar is a metric measurement of atmospheric pressure. Standard is 1013.2 millibars or 29.92 inches of mercury on a barometer. Pressure drops as a hurricane strengthens. The lowest pressure ever in the Atlantic was 882 mb in Hurricane Wilma in 2005.

What is the eye?

The calm area at the center of the storm. The eye wall surrounds the eye and has the most intense wind and weather.

What is an eyewall replacement cycle?

Very often, as storms intensify, the eyewall collapses into the center of the storm as a new eyewall forms. There is usually a brief moment of a weaker storm before the next eyewall takes over and the storm intensifies again.

Are feeder bands and spiral bands the same thing?

No. Feeders are bands of small thunderstorms or rain squalls that "feed" into the circulation of a hurricane. Spiral bands are bands of small thunderstorms or rain squalls that spiral outward from a hurricane.

How is intensity measured?

The "accumulated cyclone energy" measures the duration and intensity of storms. At each advisory when a system is at tropical storm strength or higher, the intensity is measured and then all intensity is accumulated for the entire season. A "normal" season would have an ACE of 100. The ACE for the 2010 season in the Atlantic was 165. The ACE for the 2005 season was 248.

Tropical terminology: What you need to know about hurricane season 05/17/11 [Last modified: Thursday, May 19, 2011 11:43am]

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