Thunderstorms often occur as single entities, and form when there are only light winds at all altitudes. If the wind picks up sharply and substantially changes direction, this can give rise to a so-called supercell. At their base, supercells can reach a diameter of up to 50 km, and grow to a height of over 10 km, extending above the border of the troposphere.
How supercells are formed
In order for supercells to form, wind shear is needed in addition to a fully-fledged thunderstorm. Wind shear is caused either by differing wind speeds at different altitudes, or by differing wind directions. The strong updraft causes rotating air parcels within the thunderstorm to be tilted vertically. As a result, the updraft column itself begins to rotate. If the process intensifies even further, this can result in a tornado.