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Use the glossary to get a definition to any word you might not or have questions about. The glossary covers weather words from the simple to the complex.
The NWS uses a multi-tiered system for forecasting hazardous weather. The alerts issued range from outlooks (for long lead time) to advisories and warnings (for periods of 2 days or less).
Do you know what to do when a watch, warning or advisory is issued? Learn where to go and what to do when severe weather threatens.
How does the National Weather Service issue advisories? What advisories are issued to keep you informed of hazardous and dangerous weather?
How do tornadoes form? Where and when do hurricanes form? Learn about severe weather and events in this section.
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What is a tornado?
- A tornado is a violent rotating column of air extending from a thunderstorm to the ground.
- The most violent tornadoes can produce massive destruction with wind speeds of 250 mph or more.
- Damage paths can be more than 1 mile wide and 50 miles long.
- 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.
How do tornadoes form?
- There are three basic ingredients necessary for development of a tornado.
- Moist, unstable air at the surface.
- Cold air aloft.
- Change in wind direction and/or speed with height (Vertical wind shear)
- What happens:
- 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 it 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.
How do we classify tornadoes?
- Tornadoes are classified by wind speed and damage according to the Fujita Scale:
- F0: Winds -72 MPH, Light Damage
- F1: Winds 73-112 MPH, Moderate Damage
- F2: Winds 112-157 MPH, Considerable Damage
- F3: Winds 158-206 MPH, Severe Damage
- F4: Winds 207-260 MPH, Devastating Damage
- F5: Winds 260+ MPH, Incredible Damage
Where and when do tornadoes occur?
- 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.
- Even though tornadoes can occur in the United States anywhere at any time of the year, there are certain areas that favor tornado formation at different times of the year.
- In the southern states peak tornado season is March through May when warm, moist gulf air mixes with the cooler air masses that extend into the south.
- In the northern states the peak tornado season is in the summer, when the warm, moist gulf air reaches farther north.
- In some areas along the Gulf Coast there is a second peak season in the fall.
Who is most at risk from Tornadoes?
- People in automobiles.
- The elderly, very young, and the physically or mentally impaired.
- People in mobile homes.
- People who may not understand the warning due to a language barrier
How can I tell if a tornado is about to occur?
- In addition to keeping abreast of the latest watches and warnings issued by the NWS there are certain environmental clues that Mother Nature provides us with:
- Dark, often greenish sky
- Wall cloud
- Large hail
- Loud roar-similar to a freight train
Where can I find out more information about tornadoes?
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