Tornadoes are the world’s most violent storms. The strongest winds likely approach 480 kilometers per hour (300 miles per hour), creating a fairly narrow, but catastrophic, track of destruction. While meteorologists continue to puzzle over aspects of their formation and life cycle, these rapidly whirling columns of air almost always spring from vigorous thunderstorms. Thus, a cumulonimbus, or thunderhead, is the ultimate cloud source for the majority of tornadoes.
Cumulonimbus clouds -- potential tornado factories -- are towering formations created when a heated air mass becomes unstable and rises significantly, as through strong convection or at the boundary of an oncoming front. Dependent on variations in temperature, humidity and wind direction, such an air mass may simply produce fluffy cumulus clouds, or remain buoyant enough to continue ascending. Piled-up cumulus clouds may billow into cumulonimbus, surging with a central updraft and flanking downdrafts and spitting rain, lightning and thunder. A mature cumulonimbus typically has an anvil-shaped top where it masses against a capping layer of stable air. This is often the tropopause -- the boundary between the lower atmosphere, or troposphere, and the higher stratosphere. An energetic thunderhead, however, may display “overshooting tops” -- cloud hummocks extending into the tropopause and even the stratosphere -- another clue to a storm volatile enough to generate a tornado.
A special kind of cumulonimbus formation, the exceptionally powerful thunderstorm called a supercell, seems particularly associated with the birth of tornadoes. Major instability and significant vertical wind shear -- contrasts in wind speed and direction with altitude -- are the prime ingredients for a supercell thunderstorm, which is characterized by a powerfully rotating updraft called a mesocyclone. In its movement and its mixing of warm and cold, dry and moist air masses, a mesocyclone is basically a miniature extratropical cyclone, the huge low-pressure disturbances responsible for much of the weather of the mid-latitudes. Mesocyclones may appear noticeably tilted and generally have a flattish, non-precipitating base and a train of cumulus clouds stretching, usually southwestward, behind the supercell thunderhead.
The next phase often discerned in the emergence of a tornado is a wall cloud dropping from a mesocyclone, although such clouds are sometimes observed in non-supercell thunderstorms. Frequently rotating themselves, these dark, menacing draperies are basically lowering protuberances of the mesocyclone’s base. Updrafts sucked into the wall cloud may whirl into a spinning column and produce a pre-tornado funnel cloud.
A funnel cloud is the immediate forerunner of a full-blown tornado. A funnel cloud descending from a thunderhead can wear many guises, from a slender, writhing whip to an only slightly tapering, elephantine block. Its visibility stems from condensed water vapor and, closer to the ground, inhaled and spun debris. If it touches down -- and not all do -- a funnel cloud officially becomes a tornado. A twister transitions through several phases, wreaking most of its destruction in its “mature” stages, and gradually weakening and dissipating in “decaying” and “rope” forms.
