A tornado is the most intense form of windstorm found in nature. While there are variations, the most common presentation is a dark funnel-shaped cloud, most often moving from southwest to northeast in the Northern Hemisphere. Their internal winds can exceed 480 km/hr (300 mph) and their forward speed ranges from 0 to 110 km/hr (70 mph). 
Tornadoes most often are associated with thunderstorms, although they can come from a relatively calm sky. Visual danger signs include:
- Dark, often greenish sky
- Large hail
- A large, dark, low-lying cloud, especially if rotating
- A loud roar
With thunderstorms, they may form at the "gust-front boundary where an advancing mass of cold air overruns and displaces pre-existing warmer humid air. Within a cell a strong persistent updraft of warm moist air is maintained as air enters the forward right flank at low altitude. As the air ascends it is forced to turn due to the variation of wind speed with height (known as vertical wind shear) and due to its proximity to a downdraft of drier cold air. By this means, the buoyant warm updraft acquires rotation in an anticlockwise sense as it undergoes local stretching in the vertical. The spinning, spiralling effect gradually extends along the length of the updraft, and the speed of rotation or ‘twisting’ increases as the effective column diameter diminishes."
They are most common over land, but can accompany hurricanes and tropical storms. A tornado that forms over water is called a waterspout.
In the United States, they can occur anywhere, but are most common east of the Rocky Mountains in the spring and summer. In the United Kingdom, tornadoes are most frequently reported in the Western Midlands, Eastern Midlands, Central-Southern England, South-Eastern England and East Anglia. Some occur in South-Western England, North-Western England, North-Eastern England and Wales.
Since tornado formation can be detected by radar and other instruments, the best warning is weather radio. In high-risk areas, there may also be warning sirens, sounded either when there is a radio warning or a tornado is sighted. U.S. weather reporting uses two terms:
- Tornado watch: Tornadoes are possible
- Tornado warning: A tornado has been sighted visually or by radar