Thunderstorms and Severe Weather

Table of Contents:
Information provided by the National Weather Service

   Thunderstorms and Atmospheric Conditions
   The Thunderstorm Life Cycle
   Thunderstorm Types
   Visual Aspects of Thunderstorms
   Wall Clouds and Other Lowerings
   Non-Tornadic Severe Weather Phenomena
   The Tornado
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The Tornado

What is a tornado?

The Fujita Wind Damage Scale
Classification Wind Speed
Tornadoes are classified by wind speed and damage according to the Fujita Scale
     A tornado is a violent rotating column of air extending from a thunderstorm to the ground.The most violent tornadoes can produce massive destsruction with wind speeds of 250 mph or more. Damage paths can be more than 1 mile wide and 50 miles wide. The typical tornado moves from southwest to northeast, but they have been known to move in any direction. The average forward speed of a tornado is 30 mph but it may vary from stationary to 70mph. Although tornadoes occur in many parts of the world, they are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries.

How do tornadoes form?

     Before thunderstorms develop, a change in wind direction along with an increase of wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. The area of rotation, 2-6 miles wide, now extends through much of the storm. This rotating column of air, known as a funnel extends from the cloud and grows downward toward the ground. Once the funnel touches the ground is becomes a tornado. Since the center of the funnel is a low pressure area, air rushes into the column and rises. The air is cooled as it rises and water vapor condenses to form the familiar funnel shaped cloud. As the rotating winds begin to pick up dirt and debris from the ground, the funnel will darken. The strongest tornadoes occur in supercell thunderstorms which can also produce large hail and strong downbursts.

Tornado Life Cycle

     Although not all tornadoes form from mesocyclones, most of the larger and stronger tornadoes are spawned from supercell storms with mesocyclones. Recall that a supercell's environment usually contains strong, veering winds in the lowest mile or so of the atmosphere. These strong, veering winds produce horizontal vorticity ("rolls") in the lower few thousand feet of the atmosphere. The thunderstorm's updraft then tilts these horizontal "rolls" into vertically-oriented rotation and allows the mesocyclone to form.

     The tornado circulation develops at mid levels (about 20,000 feet) in the storm where the storm's updraft and mesocyclone are strongest. The circulation gradually builds down (and up) within the storm. At about the same time, a downdraft develops at mid levels near the back edge of the storm. This downdraft, called a rear flank downdraft (RFD), descends to the ground along with the tornado circulation. Rapidly lowering barometric pressure near the ground is believed to be the primary means of drawing the tornado circulation and RFD down toward the ground. The RFD may reveal itself as a "clear slot" or "bright slot" just to the rear (southwest) of the wall cloud. Sometimes, a small shelf cloud will form along this clear slot. Eventually, the tornado and RFD will reach the ground within a few minutes of each other.

     After the tornado touches down, an ample inflow of warm, moist air continues into the tornado/mesocyclone. The RFD, though, will begin to wrap around the tornado/mesocyclone after the RFD impacts the ground. The RFD will actually cut off the inflow to the tornado as it wraps around the tornado/mesocyclone. Wind damage may result from the RFD's gust front as it progresses around the mesocyclone.

     When the RFD completely wraps around the tornado/mesocyclone, the inflow to the tornado/mesocyclone will be completely cut off. The tornado will gradually lose intensity. The condensation funnel will decrease in size, the tornado will tilt with height, and the tornado will eventually take on a contorted, ropelike appearance before it completely dissipates.

Tornado Variations

     Not all tornadoes go through the life cycle outlined above. Some tornadoes proceed from the developing stage directly to the dissipating stage, with little time spent in the mature stage. Tornadoes take on quite different appearances as they develop, mature, and decay.

     Multiple-vortex tornadoes have two or more circulations (vortices) of the storm and do not originate in mesocyclones, so in some ways they are not "legitimate" tornadoes. They can cause damage to lightweight structures and are hazardous to people in the open, though, so they do pose a threat and should be reported to the controller/dispatcher.

Thunderstorms and Atmospheric Conditions


Last Modified: Mon June 29, 1998