A rotating cone-shaped column of air extending downward from a cloud when it touches the ground it is called a tornado.
So are you asking what the cloud is called before it touches the ground? If so, I believe that the term you are looking for is "funnel cloud."
Depending on what defintion you look at, a funnel cloud becomes a tornado either A) when it touches or B) when it is about halfway between the base of the cloud and the ground.
A funnel cloud (and then a tornado) is caused by what's called a supercell thunderstorm. This kind of storm causes severe updrafts that can turn into funnel clouds.
Tornado is a violent atmospheric perturbation, with has the character of a vortex,with the aspect of a narrow column that rotates very fast or a turned funnel, reaching ground level.
When a current of cold air meets a mass of wet warm air some enormous dark clouds arise (clouds are called Cumulo nimbus).These clouds generate a thunder storm, in which warm air ascends, creating a strong current. In the upper part of the storm, strong winds begin to spin more rapidly, forming a vortex. The vortex rotates in spiral ever closer, increasing the speed and rising to the clouds then descending and striking the earth with violence.
To consider a vortex - a spiral wind funnel-shaped - as a tornado, it must be in contact with the ground and the cloud which is producing the storm. When the funnel comes in contact with the ground, a concentrated area of destruction is produced.
In 1971, Theodore Fujita - professor of meteorology at the University of Chicago, specialized in tornadoes, has made a classification of the strength of a tornado, which is called after his name and is based on human-made structures destructions. This classification is still a subjective one, even if specialists are often using to assess the intensity of tornadoes.
Fujita Scale (F - Scale) is classified as follows:
F0 - wind speed between 64-116 km / h.
Not causes very meaningful damages, but even these tornadoes can tear the roof tiles and can take cars from the roads. Can cause tearing of branches from trees and signs of road.
F1 - wind speed between 117-181 km / h.
It is a moderate tornado that causes average damages. Is equivalent to a hurricane by the weakest intensity. It will fall the roofs on houses and mobile homes, in tornado affected area, will be demolished. This type of vortex can take trains away from railways.
F2 - wind speed between 182-253 km / h.
This is a strong tornado. The heavy trees will be torn from its roots, and solid buildings will collapse as match sticks.
F3 - wind speed between 254-332 km / h.
This tornado produced damage on a large scale. It will tear down roofs and walls of houses well built. Locomotives and trucks of 400 tons will fly through the air like toys and a forest from trees will be prostrated.
F4 - wind speed between 333-419 km / h.
A tornado of such intensity destroys all that stands in their way. Solid houses are rised high in the air, and the structures with weak foundation are thrown far away.
F5 - wind speed between 420-512 km / h.
It is an incredible tornado, which is dragging all met in the way on considerable distances.It has a strength similar to that of an atomic bomb.
F6 - wind speed between 513-610 km / h.
Such a tornado is unlikely but it could have serious unimaginable consequences .Diameter of the spiral of a tornado is between several tens and several hundred meters, but sometimes registering tornadoes with much larger sizes (up to 200 to 300 km).
To remember is that a large tornado does not mean it is very strong. A large tornado may have a low intensity and vice versa.