funnel cloud

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A funnel-shaped cloud descending from a larger cloud, especially from a wall cloud, caused by the sudden condensation of water in the extreme low pressure of rapidly rotating air, as typically occurs in a tornado.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun meteorology A tapering cloud descending from a larger storm cloud, indicating the presence of an intense vortex. A tornado is a funnel cloud which reaches the ground.