Types of storms and how they form



Posted on May 21, 2010 at 3:05 PM

Tropical Cyclone: A low-pressure weather system in which the central core is warmer than the surrounding atmosphere. Storms called hurricanes or typhoons elsewhere are called tropical cyclones in the Indian Ocean and around the Coral Sea off northeastern Australia.

Tropical Depression: A tropical cyclone with maximum sustained winds near the surface of less than 39 mph.

Tropical Storm: Tropical cyclone with 39 to 74 mph winds.

Hurricane: A tropical cyclone with winds of 74 mph or more.

Typhoon: A hurricane in the north Pacific west of the International Date Line.

How They Form

Fewer than 10 percent of tropical weather disturbances grow into tropical storms because the right ingredients are relatively rare. Here's a look at the optimum conditions.

   1. Ocean water above 80°F is needed for the proper amount of water to evaporate. Warm water must be about 200 feet deep because storms stir up the ocean, bringing up cold water from below.

   2. Winds need to be coming together near the surface of the water.

   3. The air needs to be unstable so it will continue rising.

   4. Air up to about 18,000 feet needs to be humid as it's pulled into the storm. The extra water vapor supplies more latent heat energy.

   5. Pre-existing winds, those not created by the storm- should be coming from nearly the same direction and at close to the same speeds at all altitudes to avoid ripping the storm apart.

   6. An upper atmosphere high-pressure area helps pump away air rising in the storm.

Saffir-Simpson Scale

Hurricanes not only have names, they have numbers based on the Saffir-Simpson Damage Potential Scale. The name stems from Herbert Saffir, a consulting engineer in Coral Gables, Florida, and Robert Simpson, then director of the National Hurricane Center, who developed the scale in the early 1970's.

The center was having difficulty telling disaster agencies how much damage to expect from particular storms. So Simpson called on Saffir, and they approached it from an engineering point of view. The following is what they came up with.

  Cat. 1 Cat. 2 Cat. 3 Cat. 4 Cat. 5
  Minimal damage Moderate damage Extensive damage

Extreme damage

Catastrophic damage
Wind speeds 74-95 mph 96-110 111-130 131-155 155+
Barometric pressure More than 28.94 in. 28.50 - 28.91 27.91 - 28.47 27.17 - 27.88 Less than 27.17
Storm surge 4-5 ft. 6-8 9-12 13-18 18+