Sample Page: Glossary

Glossary entry for "tropical cyclone."

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Graphics in the glossary are presented as hotwords beginning with "G" and also as thumbnails below the text. Two samples are given below:

In this example, we have turned on both options (show winds and show pressures) to show how you can interact with the graphics, although using one at a time would be less cluttered. This graphic, as to most, includes extensive notes in the way of an extended figure caption.

Caption to graphic G190
"No two hurricanes or typhoons are the same, but they do have similar general properties. This picture is intended to represent average values for a larger system. Note the correlation in clouds, squalls, wind, and barometer which are shown here very schematically.

This one shows winds at 100 miles off of some 50 to 70 knots; in smaller such systems, which may be more typical, the winds could be as low as 40 knots or so at 80 to 100 miles off, when the systems are still below some 20 latitude. At higher latitudes after recurvature the region of strong winds often expands considerably. The very strong winds are concentrated near the wall cloud within some 50 miles of the center. It obviously prudent to keep alert early and put as much distance as possible between you and the center.

The time scale on the pressure trace assumes a storm motion of some 300 miles in 24 hours (a relative speed of 12.5 knots).

The dotted line along the clouds represents roughly the outline that would appear as the cloud bar on the horizon. This could be in clear sight while conditions at your position are still relatively normal, with still adequate time to maneuver out of the path of the most intense winds and seas.

Within this schematic picture, once the barometer starts down (5 mb below the normal for the region, season, and time of day, after perhaps a slight rise) then it is clear sign of the presence of a tropical cyclone, even if a cloud bar is not discernible.

See ART-25 on tropical cyclones. See also G245 on tropical cyclones."

Sample Glossary entry
Blue words in the program text (not here) are linked to the Glossary, graphics, or other parts of the program.

tropical cyclone

The general term for cyclones originating in the Tropics or Subtropics. In successive stages of intensification, the tropical cyclone may be classified as tropical disturbance, tropical depression, tropical storm, and hurricane or typhoon. Statistics on the locations and frequency of these storms are given in G193. See also cyclone.

Tropical cyclones are classified by form and intensity as follows:

A tropical disturbance is a discrete system of apparently organized convection generally 100 to 300 miles in diameter having a nonfrontal migratory character, having maintained its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. It has no strong winds and no closed isobars. i.e., isobars that completely enclose the low

A tropical depression has one or more closed isobars and some rotary circulation at the surface. The highest sustained (1-minute mean) surface wind speed is 33 knots.

A tropical storm has closed isobars and a distinct rotary circulation. The highest sustained (1-minute mean) surface wind speed is 34 to 63 knots.

A hurricane or typhoon has closed isobars, a strong and very pronounced rotary circulation, and a sustained (1-minute mean) surface wind speed of 64 knots or higher. The maximum winds of a cyclone can be roughly estimated from the central pressure (Po) as V (knots) = 15 x square root(1010-Po). Questions in the topic Lows and hurricanes present further discussion."

A super typhoon is a typhoon with sustained winds over 130 knots.

Tropical cyclones occur almost entirely in six rather distinct areas, four in the Northern Hemisphere and two in the Southern Hemisphere. The name by which the tropical cyclone is commonly known varies somewhat with locality as follows (see G193):

North Atlantic: A tropical cyclone with winds of 64 knots or greater is called a hurricane.

Eastern North Pacific: The name hurricane is used as in the North Atlantic.

Western North Pacific: A fully developed storm with winds of 64 knots or greater is called a typhoon or, locally in the Philippines as a Baguio.

North Indian Ocean: A tropical cyclone with winds of 34 knots or greater is called a cyclonic storm.

South Indian Ocean: a tropical storm with winds of 34 knots or greater is called a cyclone.

Southwest Pacific and Australia area: the name cyclone is used as in the South Indian Ocean.

A severe tropical cyclone originating in the Timor sea and moving southwestward and then southeastward across the interior of northwestern Australia is called a willy-willy. Tropical cyclones have not been observed in the south Atlantic Ocean or in the South Pacific Ocean east of longitude 140 W.

See G190 for an illustration of pressure and wind speed within a tropical cyclone. G189 is a comparison of tropical and extratropical cyclones. G191 illustrates the formation of a tropical cyclone by an easterly wave in the Intertropical Convergence Zone.

G193 lists statistics by region and month for hurricane occurrence worldwide. G194 shows the characteristics of cyclone tracks in the Northern Hemisphere. South of the equator, they move west and then curve poleward toward the south.

Another sample graphic (G189) is shown below, followed by the full text that it includes... a comparison of severe storms in and out of the Tropics.


Here is the full text included in the above graphic:
Tropical Storms Extratropical Storms
Form in the Tropics Form outside of the Tropics
Tropical cyclones are primarily oceanic, forming and maturing only over warm oceanic waters. If they cross over large land areas, they weaken and eventually die. Extratropical cyclones, on the other hand, may develop and intensify over water or land.
Tropical cyclones develop primarily in the summer and autumn months of their hemispheres. Extratropical cyclones occur at any time of year, but are most powerful during the winter.
For some portion of their lives, most tropical cyclones move in a westerly direction. Extratropical cyclones generally head eastward and rarely travel toward the west.
Mature tropical cyclones are generally much smaller than extratropical cyclones. A typical tropical storm, for example, is 200 to 500 miles in diameter. A North Pacific winter cyclone, on the other hand, may span 1500 miles or more. Typical extratropical storms are some 700 to 1,000 miles in diameter.
Tropical cyclones do not have fronts. Extratropical cyclones usually have associated frontal systems.
Mature tropical cyclones have a central calm area called the "eye". Extratropical storms do not have an eye.
The strongest winds in a tropical cyclone are near the center. The strongest winds in an extratropical cyclone are usually near the periphery.
Mature tropical cyclones may have sustained winds of 120 to 150 knots -- a "super typhoon" is one with winds over 130 knots. Extratropical storms are almost never this intense, although very severe ones can generate winds up to 100 knots and they are often over 64 knots which defines hurricane strength.
Tropical cyclones have "warm cores", i.e. the temperature in the center of the storm is higher than it is on the periphery. Extratropical cyclones have cold cores.
Tropical cyclones are relatively rare. In a typical year, about 70 tropical storms of hurricane strength will form worldwide. In the same year, there would be some 1,500 or more extratropical storms developing.