Funnel clouds are captivating atmospheric phenomena often observed during severe weather. These swirling columns of air frequently lead to misunderstandings among the general public. Their nature and the processes that create them are distinct from other severe weather events.
What is a Funnel Cloud?
A funnel cloud is a rotating column of wind that extends downward from the base of a cloud, typically a cumulonimbus or towering cumulus cloud. It is characterized by its distinctive funnel-shaped or rope-like appearance. A funnel cloud does not make contact with the ground or a water surface.
The visible part of a funnel cloud consists of condensed water droplets. Air drawn into the low-pressure vortex within the rotating column cools as it expands. If the air cools to its dew point, the water vapor condenses, making the spinning column visible. The funnel becomes more defined as condensation occurs.
How Funnel Clouds Form
Funnel clouds generally form within severe thunderstorms, most often supercells, which are characterized by a rotating updraft called a mesocyclone. The genesis of this rotation often involves wind shear, where there is a change in wind speed or direction with height. This wind shear can create a horizontal “tube” of spinning air.
As this horizontal rotation is drawn into the thunderstorm’s strong updraft, it can be tilted vertically, forming the mesocyclone. Atmospheric instability, with warm, moist air near the surface and cooler air aloft, also plays a role by fueling the strong updrafts necessary for thunderstorm development.
Funnel Cloud vs. Tornado
The primary distinction between a funnel cloud and a tornado lies in whether the rotating column of air makes contact with the Earth’s surface. A funnel cloud is a rotating column that does not touch the ground. It is a visual precursor that indicates the potential for a tornado, but it is not yet a tornado.
A tornado is confirmed when the violently rotating column of air extends from the base of a storm cloud and makes contact with the ground. This ground contact allows a tornado to pick up debris, dust, and other materials, creating a swirling effect at its base. While a funnel cloud signals dangerous atmospheric conditions, treat any observed funnel cloud with caution, as it could quickly become a tornado.