How are tornadoes formed? 

Tornadoes are formed by air rising quickly from thunderclouds. When that rising air hits air that is moving, it turns to rotate in different directions. This rising air goes faster and faster. The column grows and grows, longer and larger and eventually touches the ground. It is very loud and noisy. To make a Tornado it  needs cool dry air and warm moist air. The warm moist air is lighter then dry cold air. That is how you can tell the difference.

Diagrams

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Types of Tornadoes

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There are different kinds of tornadoes.
A Supercell tornado is mostly violent (dangers). They are formed from lower clouds.
Land spout forms along the edges of cool air downdraft of thunderstorms. It's weaker then supercall tornadoes.
Gustnado twisters are very weak and it forms in gust in front the thunderstorm.
Wedge ones are shaped straight not shaped funnel. They'er wider then taller (shorter but wider). Wedges are not too strong but it covers more ground (it touches more ground).
Fire whirls are created by a major forest and volcanic eruptions.  

How do tornadoes effect land and humans

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Twisters or tornadoes effect humans in many different ways. Some people die, others can not see where they are going and properties and houses can get destroyed. It effects the envioment because there are no trees which have a negative effect on oxygen. After a tornado many people find difficult to breath due to a luck of oxygen.