Tornados

Tornado!

Greg Stumpf

Although tornadoes occur in many parts of the world, these destructive forces of nature 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. A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas!

Tornadoes Occur Anywhere

Tornado Myths:

MYTH: Areas near rivers, lakes, and mountains are safe from tornadoes.
FACT: No place is safe from tornadoes. In the late 1980's, a tornado swept through Yellowstone National Park leaving a path of destruction up and down a 10,000 ft. mountain.

MYTH: The low pressure with a tornado causes buildings to "explode" as the tornado passes overhead.
FACT: Violent winds and debris slamming into buildings cause most structural damage.

MYTH: Windows should be opened before a tornado approaches to equalize pressure and minimize damage.
FACT: Opening windows allows damaging winds to enter the structure. Leave the windows alone; instead, immediately go to a safe place.

Carolinas Outbreak:

March 28, 1984, afternoon-evening

22 tornadoes

57 deaths

1,248 injuries

damage $200 million

37% of fatalities in mobile homes

Pennsylvania-Ohio Outbreak:

May 31, 1985, late afternoon-evening

41 tornadoes, including 27 in PA and OH

75 deaths in U.S.

1,025 injuries

damage $450 million

Plains Outbreak:

April 26-27, 1991, afternoon of 26th through early morning 27th

54 tornadoes

21 deaths

308 injuries

damage $277+ million

15 deaths in/near mobile homes, 2 deaths in vehicles

Frequency of Tornadoes

Tornadoes can occur at any time of the year.

In the southern states, peak tornado occurrence is in March through May, while peak months in the northern states are during the summer.

Note, in some states, a secondary tornado maximum occurs in the fall.

Tornadoes are most likely to occur between 3 and 9 p.m. but have been known to occur at all hours of the day or night.

The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction. The average forward speed is 30 mph but may vary from nearly stationary to 70 mph.

The total number of tornadoes is probably higher than indicated in the western states. Sparce population reduces the number reported.

How Do Tornadoes Form?

Before thunderstorms develop, a change in wind direction and an increase in 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.

An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

Woodward OK (Ron Przybylinski)

A lower cloud base in the center of the photograph identifies an area of rotation known as a rotating wall cloud. This area is often nearly rain-free. Note rain in the background.

Woodward OK (Ron Przybylinski)

Moments later a strong tornado develops in this area. Softball-size hail and damaging "straight-line" winds also occurred with this storm.

http://youtube.com/watch?v=LbiX0aaYm3I watch video

Tornado Variations

Some tornadoes may form during the early stages of rapidly developing thunderstorms. This type of tornado is most common along the front range of the Rocky Mountains, the Plains, and the Western States.

Tornadoes may appear nearly transparent until dust and debris are picked up.

Occasionally, two or more tornadoes may occur at the same time.

Waterspout

Waterspouts are weak tornadoes that form over warm water.

Waterspouts are most common along the Gulf Coast and south eastern states. In the western United States, they occur with cold late fall or late winter storms, during a time when you least expect tornado development.

Waterspouts occasionally move inland becoming tornadoes causing damage and injuries.

Colorado Tornado (David Blanchard)

Dr. Joseph Golden (NOAA)

These dangers often accompany thunderstorms:

Flash Floods: Number ONE weather killer - 146 deaths annually

Lightning: Kills 75-100 people each year

Damaging Straight-line Winds: Can reach 140 mph

Large Hail: Can reach the size of a grapefruit - causes several hundred million dollars in damage annually to property and crops

Contact your local National Weather Service office, American Red Cross chapter, or Federal Emergency Management Agency office for a copy of the "Thunderstorms and Lightning...The Underrated Killers" brochure (NOAA PA 92053) and the "Flash Floods and Floods...The Awesome Power" brochure (NOAA PA 92050).