No one knows exactly what causes tornadoes, but the big question currently is if climate change is influencing tornado occurrences, according to the National Severe Storms Laboratory, part of the National Oceanic and Atmospheric Administration.

There isn’t enough evidence to determine if the number or severity of tornadoes and other storm aspects such as hail, lightning and dust storms show trends, according to the National Science and Technology Council’s Scientific Assessment on Climate Change. That’s because tornadoes last seconds and minutes and climate trends take many years, decades and longer, spanning the globe. The council said there are too many unknowns.

Climate models can show broad-scale shifts in several things that go into causing severe thunderstorms, such as moisture, instability and wind shear, but having some weather conditions favorable to tornadoes doesn’t guarantee there will be a tornado.

Key tornado ingredients depend on “day-to-day patterns, and often, even minute-to-minute local weather,” the laboratory indicated.

Also, tornado records don’t exist in most parts of the world and only started to be kept in the United States in 1950.

The National Severe Storms Laboratory also indicated that El Nino and La Nina conditions don’t cause tornadoes because both are major changes in sea surface temperatures in the Pacific Ocean that happen over a span of months.

Fast-acting storms

Tornadoes in the United States happen in seconds and minutes.

El Nino affects weather patterns on a large scale, according to the laboratory. But there are too many variables to say for certain if El Nino or La Nina have anything to do with tornado risk.

“There are climate shifts and I think we’re in a warming trend, but there’s no way to tell if that directly affects the formation of tornadoes,” said Matt Bishop, meteorologist with the National Weather Service.

Supercells are associated with the most destructive and deadly tornadoes.

Supercells are rotating thunderstorms with a mesocyclone, a well-defined circulation. What is believed to cause tornado formation is what happens in and around the mesocyclone, the laboratory theorized. And because of recent research gathered through the VORTEX2 program, it appears the development of a tornado is related to temperature differences across the edge of air around the mesocyclone.

But other research shows tornadoes can form without those temperature patterns too, according to the Laboratory.

2011 had more than the usual number of yearly tornadoes in the United States with 1,691 reported, which was more than any year on record except for 2004, which had 1,817 tornadoes. April 2011 had the greatest number of tornadoes in a single month with 758 and the largest daily total of 200 on April 27, according to NOAA.

About 1,200 tornadoes strike in the United States each year.

But the United States isn’t the only location of tornadoes. Tornadoes happen in Australia, Europe, Africa, Asia and South America, according to NOAA. New Zealand has about 20 tornadoes every year. Argentina and Bangladesh have two of the highest concentrations of tornadoes outside of the United States.

Most tornadoes happen in Texas between 4 and 9 p.m. and worldwide between 3 and 7 p.m., with a peak near 5 p.m. What affects tornado occurrence is solar heating, according to David Cook, meteorologist with the Climate Research Section at the Argonne National Laboratory in Chicago. Later in the afternoon, the warmer air rises from the surface, resulting in more frequent, larger and intense thunderstorms that have the possibility of spawning more frequent and more intense tornadoes, according to Cook.

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