How Do Tornadoes Form?

All tornadoes start from thunderstorms, but as certain conditions are met, a rotating funnel forms and a tornado is born.

Thunderstorms start when hot air rises from the ground (thermals) and causes clouds to rise. As the air cools with the higher elevation, water condenses and forms droplets. The water condenses and heat is produced, heating the air and causing an updraft of air. This updraft causes a low pressure area beneath the thunderstorm. 

When the clouds change into cumulonimbus clouds, the water droplets start to merge with one another, the clouds become heavier, and water begins to freeze.  As the ice falls, it melts and turns to rain which produces a downdraft. This turbulence between updrafts and downdrafts creates wind, positive and negative charges make lightning, and tornadoes may possibly form.

Lightning and thunder are a big part of a thunderstorm. As the turbulence increases inside the storm, the water droplets and ice crystals break up and become electrically charged. The upper part of the cumulonimbus clouds is positively charged, and the middle and bottom portion of the cloud is negatively charged. Lightning happens when a discharge occurs between the cloud and the earth’s surface. When the lightning heats the air, it expands quickly, and the rumbling of thunder is created.    

Tornado Formation

Before a thunderstorm forms, there is an area in the lower atmosphere of horizontal, spinning air caused by a change in wind direction and an increase in the wind’s speed along with an increase in height. When the thunderstorm starts to develop, the updraft lifts this area of spinning air, changing its pitch from horizontal to vertical. At this point, there is a large area of rotation, from two to six miles in diameter, where the tornado typically forms. A wall cloud will form, a low mass of clouds which rotates. A tornado forms, which is accompanied by high winds and damaging hail.

These dark green clouds that indicate a possibly tornadic storm occur often in Tornado Alley. This is an area where many tornadoes are formed, and includes parts of Nebraska, Kansas, Oklahoma, and Texas. One thing that contributes to tornadoes is unstable air. When warm air rises, the atmosphere is considered unstable. When the air is cooler than the surrounding air, it will sink and the atmosphere is said to be stable. A thunderstorm cannot develop in stable conditions.

Supercells

Supercells are very large, rotating thunderstorms. They can live a very long time and spawn several tornadoes. Tornadoes formed from supercell thunderstorms are more damaging partly because of their duration. One kind of supercell creates huge amounts of precipitation and large, damaging hail. The other kind produces less precipitation and large hail but makes tornadoes. Squall lines form from the border between warm and cold fronts or from a storm that has split. 

Most of the precipitation in the United States comes from a squall line. One kind of squall line that can develop is a line of supercell storms. Sometimes a squall liner can reach over 600 miles in length. So a further answer to, “How do tornadoes form?” is they can form in a squall line of supercells, and each supercell can form several tornadoes.    

Watches and Warnings

A tornado watch means that conditions are favorable for the formation of a tornado. It implies a severe thunderstorm warning, as any thunderstorm capable of producing a tornado will be severe. Usually, there will also be a threat for large hail and high winds. When a watch is issued, stay alert to changing weather conditions, especially looking for dark green clouds or orange-gray skies, hail, rotating clouds or funnels, and listen for a loud roar. Also, stay tuned to local weather updates and be ready to take action quickly.  

A tornado warning means that either storm spotters have seen a funnel, or radar is indicating a possible funnel. Tornado sirens may sound in your area. You will need to take shelter immediately and listen for updates. If you are outside, go to a low-lying area, like a ravine or ditch and cover your head with your hands. Do not take shelter under an overpass. If you are in a trailer or car, seek shelter in a building immediately. If you are in a building, go to the basement or lowest floor and go to the center of an interior room, like a closet, bathroom, or hallway that is away from windows, doors, corners, and outside walls.