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Spartanburg County Office of Emergency Management

 

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Hurricane FAQ's

 

What is a Hurricane?

How are Hurricanes different from Tornados?
 How does a Hurricane form?
 What is a Hurricane Watch?
 What is a Hurricane Warning?
 

Answers

What is a Hurricane

The term "hurricane" is a specific name for a strong "tropical cyclone". A tropical cyclone is the generic term for a low-pressure system over tropical waters with organized thunderstorm activity and defined  wind circulation.  Tropical cyclones with maximum sustained winds of less than 39 mph (34 kts) are called "tropical depressions".  Once the tropical cyclone reaches winds of at least 39 mph, they are typically called a "tropical storm" and assigned a name.  If winds reach 74 mph (64 kts) then they are called a hurricane.
How are Hurricanes different from Tornadoes?

While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of feet and are produced from a single storm (i.e. a thunderstorm).  A tropical cyclone, however, has a diameter on the scale of miles and is comprised of several to dozens of convective storms.   Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex.   In contrast, tropical cyclones are purely an oceanic phenomena - they die out over-land due to a loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes. 
 How does a Hurricane Form?

A hurricane needs several different factors present in order for it to develop.

  1. Warm ocean waters of at least 80°F throughout a sufficient depth  of at least 150 ft. Warm waters are necessary to fuel the heat engine of the tropical cyclone.
  2. An atmosphere which cools fast as the elevation increases.  This causes instability in the accumulation of moisture.  
  3. Relatively moist layers near the mid-troposphere (about 3 miles up). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.
  4. A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.

Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop. 

What is a Hurricane Watch?

A hurricane watch is issued when there is a threat of hurricane conditions within 36 hours. 
 

What is a Hurricane Warning?

A hurricane warning is issued when hurricane conditions
(winds of 74 miles per hour or greater, or dangerously high water and rough seas) are expected in 24 hours or less.