Estimates of Hurricane Intensity

A tropical cyclone (the generic name for hurricane-type storms) is identified in the historical records by its name or number, order of occurrence during the season, ocean basin, the date range of its existence as a tropical cyclone, and the intensity classification or category representing its estimated top sustained surface wind speeds at any point during its existence.

If top wind speeds are less than 39 mph but a closed circulation of winds around a center of low pressure is clearly evident, the system may be categorized as a tropical depression. Not yet worthy of a name, the system is nonetheless recognized for historical purposes as a tropical cyclone and assigned a permanent unique number.

If those winds are at least 39 mph but below 74 mph, the system is categorized as a tropical storm and given its name. If the storm is found to have attained top winds in excess of 74 mph, that system has now become a hurricane. Anything above 155 mph and the system is designated a Category-5 hurricane; this is the maximum intensity category for tropical cyclones in the Atlantic basin.

The intensity categories currently in use, together with their wind speed ranges and associated symbols, are listed in the table. Certain central pressure ranges and storm surge ranges are also broadly associated with each category storm, though these may vary.

Saffir-Simpson Hurricane Intensity Scale with Entries for Tropical Storm and Tropical Depression

Symbol	Category	Wind Speed (Maximum 1-minute sustained)
	Tropical Depression	below 39 mph (with closed circulation around low pressure)
	Tropical Storm	39-73 mph
	Category-1 Hurricane	74-95 mph
	Category-2 Hurricane	96-110 mph
	Category-3 Hurricane	111-130 mph
	Category-4 Hurricane	131-155 mph
	Category-5 Hurricane	above 155 mph

An area of disturbed weather without the minimum intensity or concentric wind structure of a tropical depression (closed surface low) but nonetheless visible to forecasters is identified as a "tropical disturbance" In this case, the winds flowing around the low pressure do not form a closed circle, but an open flow more closely resembling a wave. If the disturbance strikes forecasters, based on its internal level of organization and the amicability of the surrounding environment, as capable of development into a tropical cyclone and deemed worthy of further investigation or attention, it is tagged with a recyclable number denoting it as an INVEST or investigative area.

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