What must the wind speed be for a tropical storm to officially become a hurricane?

[This message was edited by Kelleygirl on 09-19-03 at 10:00 PM.]

If it strengthens with sustained winds in excess of 39 mph but less than 74 mph, it is a "tropical storm."

SAFFIR/SIMPSON SCALE
All hurricanes are dangerous, but some are more so than others. The way storm surge, wind, and other factors combine determines the hurricanes destructive power. To make comparisons easier and to make the predicted hazards of approaching hurricanes clearer to emergency forces-National Oceanic and Atmospheric Administration's hurricane forecasters use a disaster-potential scale which assigns storms to five categories.

Hurricane Categories
CATEGORY -Definition - Effects
ONE
Winds 74-95 mph: No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal road flooding and minor pier damage.

TWO
Winds 96-110 mph: Some roofing material, door, and window damage to buildings. Considerable damage to vegetation, mobile homes, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of center. Small craft in unprotected anchorages break moorings.

THREE
Winds 111-130 mph: Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain continuously lower than 5 feet ASL may be flooded inland 8 miles or more.

FOUR
Winds 131-155 mph: More extensive curtainwall failures with some complete roof strucutre failure on small residences. Major erosion of beach areas. Major damage to lower floors of structures near the shore. Terrain continuously lower than 10 feet ASL may be flooded requiring massive evacuation of residential areas inland as far as 6 miles.

FIVE
Winds greater than 155 mph: Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Major damage to lower floors of all structures located less than 15 feet ASL and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5 to 10 miles of the shoreline may be required.

And while I am at it:

FUJITA SCALE
The Fujita Scale is used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structure. The Fujita Scale(also known as the Fujita-Pearson Scale) may not be a perfect system for linking damage to wind speed, but it had distinct advantages over what had gone on before its inception. And it was simple enough to use in daily practice without involving much additional expenditure of time or money. From a practical point of view, it is doubtful that any other system would have found its way into widespread accepted use, even to this day. The entire premise of estimating wind speeds from damage to non-engineered structures is very subjective and is difficult to defend from various meteorological perspectives. Nothing less than the combined influence and and prestige of the late Professor Fujita and Allen Pearson, director of NSSFC(National Severe Storm Forecast Center) in 1971 could have brought this much needed system into widespread use. The FPP scale rates the intensity of the tornado, and measured both the path length and the path width. The Fujita part of the scale is as follows:
The Fujita ScaleF-
F0 Gale tornado 40-72 mph Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted trees; damages sign boards.

F1 Moderate tornado 73-112 mph The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.

F2 Significant tornado 113-157 mph Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles generated.

F3 Severe tornado 158-206 mph Roof and some walls torn off well constructed houses; trains overturned; most trees in fores uprooted

F4 Devastating tornado 207-260 mph Well-constructed houses leveled; structures with weak foundations blown off some distance; cars thrown and large missiles generated.

F5 Incredible tornado 261-318 mph Strong frame houses lifted off foundations and carried considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel re-inforced concrete structures badly damaged.

F6 Inconceivable tornado 319-379 mph These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies

Yep, at 74 mph, they become hurricanes.