A tornado begins in a severe thunderstorm called a supercell. A supercell can last longer than a regular thunderstorm. The same property that keeps the storm going also produces most tornadoes. The wind coming into the storm starts to swirl and forms a funnel. The air in the funnel spins faster and faster and creates a very low pressure area which sucks more air (and possibly objects) into it. Tornadoes can form any time during the year, but most form in May. But, more severe ones form earlier because the most damge is caused in April. The more north you go, the later the peak tornado season is. This is because it takes longer to warm the northern parts of the plains so tornadoes form later.
http://www.windows2universe.org/earth/Atmosphere/tornado/formation.html
The severe thunderstorms which produce tornadoes form where cold dry polar air meets warm moist tropical air. This is most common in a section of the United States called Tornado Alley. Also, the atmosphere needs to be very unstable.
http://www.windows2universe.org/earth/Atmosphere/tornado/formation.html
According to the Glossary of Meteorology (AMS 2000), a tornado is a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud..
http://www.spc.noaa.gov/faq/tornado/
Related Content Animation: How Tornadoes Form The more common tornadoes have wind speeds of less than 110 miles (177 kilometers) per hour, are about 250 feet (76 meters) across, and travel only a few miles before they dissipate.
http://news.nationalgeographic.com/2015/05/150511-tornadoes-storms-midwest-weather-science/
The average first-tornado date in the U. S. is January 11, for the entire 1950-2011 time frame. The latest first-tornado was on 15 February 2003, in Marengo County, AL, meaning that the nation had 45 days of tornado-free weather to start that year.
http://www.spc.noaa.gov/faq/tornado/
Because: Nobody knows the true wind speeds at ground level in most tornadoes, and The amount of wind needed to do similar-looking damage can vary greatly, even from block to block or building to building, ...damage rating is (at best) an exercise in educated guessing.
http://www.spc.noaa.gov/faq/tornado/
Please try again later. View full lesson: http://ed.ted.com/lessons/how-do-torn... Tornadoes are the most violent storms on Earth, with wind velocities that can exceed 200 miles per hour. How do these terrifying cyclones form? Meteorologist James Spann sheds light on the lifespan of tornadoes as they go from supercell thunderstorms to terrible twisters before eventually dissolving back into thin air. Lesson by James Spann, animation by Provincia Província. studio
http://www.youtube.com/watch?v=lmWh9jV_1ac
NOTE: Tornado climatology information used here may come from either the SPC database or Grazulis (1993). 1  Rapid cloud-base rotation, if you are close enough to make out cloud movement, and. 2  A concentrated, whirling debris or dust cloud at ground level under the thunderstorm base.
http://www.spc.noaa.gov/faq/tornado/
A tornado lowers to the ground near Chickasha, Oklahoma, on May 6. May and June tend to have more tornadoes than any other months in North America. A tornado lowers to the ground near Chickasha, Oklahoma, on May 6.
http://news.nationalgeographic.com/2015/05/150511-tornadoes-storms-midwest-weather-science/
How Tornadoes Form The most intense tornadoes emerge from what are called supercell thunderstorms. For such a storm to form, you first need the ingredients for a regular thunderstorm, says Brooks. Those ingredients include warm moisture near the surface and relatively cold, dry air above. The warm air will be buoyant, and like a hot-air balloon it will rise, says Brooks. Ron Johnson, Peoria Journal Star/AP.
http://news.nationalgeographic.com/2015/05/150511-tornadoes-storms-midwest-weather-science/
what forms a tornado
Thunderstorm called a super cell forms a tornado. A tornado forms from thunderstorms called a super cell.