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What are tornadoes?

By: Juliana Rocha

Multiple vortexes tornado*

Multiple vortexes tornado*

Tornadoes are the worst kind of storm known to man. They occur when a rotating column of air is in simultaneous contact with a rain cloud and the ground. Winds in this column might blow as fast as 300 mph! With such strong winds, you’ve probably concluded that it isn’t a good idea to stay in the path of a tornado!

Meteorologists say that both tornadoes and hurricanes are “atmospheric vortexes”. Weird name, isn’t it? But atmospheric vortexes are nothing more than very strong whirlwinds! But although the two phenomena are nothing but strong whirlwinds, tornadoes and hurricanes have little in common.

Tornadoes are around 300 feet in diameter. Hurricanes, on the other hand, can be over 60 miles across. While tornadoes are formed of a single rain cloud, hurricanes are formed of dozens of them. Unlike hurricanes, tornadoes might also have many vortexes. Besides that, tornadoes occur mostly over the ground, whereas hurricanes are only formed over hot seawater (when reaching the ground, they can’t find moisture - their main source of power - and they loose strength). Despite the differences, hurricanes might turn into tornadoes: that usually happens when a hurricane touches the ground while the weather is very bad.

Although possible anywhere in the planet, tornadoes are most common in the Rocky Mountains, in the United States, during the spring and summer. May 2003, for instance, was the record month in the number of tornadoes: 546 were registered throughout the Unites States in that month.

Supercell**

Supercell**

How is a tornado formed?

Meteorologists are still trying to understand how tornadoes are formed. The Earth’s most destructive tornadoes emerge from supercells, a kind of thunderstorm that moves in circles. Scientists believe that what causes the tornado can be in most part explained by events taking place inside and around storms. Supercells also cause lightning, hail and floods.

When tornadoes pass through vegetation or a construction site, they tear off pieces and carry them around. It is this bundle of dust, haze and debris that gives the column of air its dark tone.

Have you ever watched the movie Twister? Do you remember that a cow flies over a truck because of the wind? Well, there are a lot of weird stories concerning tornadoes...
Although they might last for up to an hour, most tornadoes are less than 10 minutes long. Therefore, despite the high wind speed, the destruction caused by a tornado is much smaller.

Classification of tornadoes

The intensity of tornadoes are measured by the Fujita scale, which takes into account the damage caused by winds on construction and vegetation. This scale ranges from zero to five. Around 70% of recorded tornadoes are considered weak, having been registered as F0 or F1, according to average wind speed. Only 2% of them reach the highest levels of destruction power, with wind speeds greater than 250 mph! Scary, isn’t it?!

 Funnel cloud***

Funnel cloud***

A F0 tornado has winds up to 72 mph and is capable of rooting small trees and damaging chimneys and signboards. F1 tornadoes have winds that vary between 73 and 112 mph, may cause damage to roof tiles and throw cars off the streets. The winds of an F2 tornado vary between 113 and 157 mph and cause severe damage to roofs, overturn automobiles and trains and throw light objects across small distances. Winds of F3 tornadoes range between 158 and 206 mph and are able to root large trees, as well as drag and throw cars. With winds extending from 207 to 260 mph, F4 tornadoes are capable of seriously damaging the foundations of houses. Thanks to their winds ranging between 261 and 318 mph, F5 tornadoes completely destroy houses and buildings and toss heavy objects such as cars and large blocks of concrete across 300 feet.

Tornadoes are still identified by the place and date in which they occur. Therefore, “Miami Lakes FL 29 OCT 2003”, for instance, is the name of a tornado that hit the city of Miami, in the American state of Florida, on October 29th, 2003. “Missouri Valley/Corn Belt 24 MAY 2004” is another example: this tornado hit a region in the United States called the “Corn Belt”, which encompasses the states of Missouri, Nebraska, Kansas and Iowa on May 24th, 2004.

Images

* Wichita Falls, Texas. April 10, 1979. Photo Winston Wells/NOAA Photo Library.

**Chaparral, New Mexico. April 3, 2004. Photo: Greg Lundeen/NOAA Photo Library.

***Union City, Oklahoma. May, 24, 1973. Photo: NOAA Photo Library.

Back to the main article:

Cyclones, hurricanes, tornadoes...

See also:

What are hurricanes?

What are cyclones and tropical storms?

Sources:

Tropical Cyclone Programme - World Meteorological Organization (TCP/WMO)

National Oceanic and Atmospheric Administration (NOAA)

Hurricane Research Division - Atlantic Oceanographic and Meteorological Laboratory (HRD/AOML/NOAA)

The Online Tornado FAQ - Storm Prediction Center (SPC/NOAA)

NOAA Photo Library  

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