A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud base. Funnel clouds form most frequently in association with supercell , and are often, but not always, a visual precursor to . Funnel clouds are visual phenomena, but these are not the vortex of wind itself.
"Classic" funnel clouds
Funnel clouds result from the low found within tornadoes. The low air pressure causes air flowing towards the vortex to expand and cool. If the air is sufficiently moist and cools to the dew point, a funnel cloud is produced. The air pressure around the outer edge of the funnel cloud generally corresponds to the air pressure of the cloud base of the parent cloud.
Debris swirls are usually evident prior to the condensation funnel reaching the surface. Some tornadoes may appear only as a debris swirl, with no obvious funnel cloud extending below the rotating cloud base at any time during the Tornadogenesis. The surface level vortex tends to strengthen over time following initial formation, making the debris swirls and the condensation more apparent.
In cloud nomenclature, any funnel- or inverted-funnel-shaped cloud descending from cumulus or cumulonimbus clouds is technically described as an accessory feature called tuba. The terms tuba and funnel cloud are nearly but not exactly synonymous; a wall cloud, for example, is also a form of tuba. Funnel clouds associated with supercells usually form within and under wall clouds.
Cold-air funnel clouds
Unlike the related phenomenon associated with severe thunderstorms, cold-air funnels are generally associated with partly cloudy skies in the wake of , especially associated with certain low pressure systems, or in association with atmospheric boundaries such as lake breeze and or outflow boundary.
The larger scale weather conditions are characterized by particularly cold air aloft over relatively warmer low level air, leading to high , and often by high environmental vorticity yet relatively meager vertical wind shear. The funnels develop where atmospheric instability and moisture are sufficient to support towering cumulus clouds but typically limited to no or to little precipitation. Cold-air funnels, although weak, may persist for several minutes, and areas of intermittently forming funnel clouds may occur for tens of minutes.
Multiple such areas of activity may form within the same region during afternoon heating. Cold-air funnels appears to be a diurnal phenomenon, weakening and eventually dissipating with loss of insolation. When precipitation does develop, the associated downdraft tends to cause rapid demise of the cold-air funnels. The mixing of cooler air in the lower troposphere with air flowing in a different direction in the middle troposphere causes the rotation on a horizontal axis, which, when deflected and tightened vertically by convective , forms a vertical rotation that can cause condensation to form a funnel cloud.Cooley J. R., and M. E. Soderberg, 1973: Cold air funnel clouds. NOAA Tech. Memo. NWS CR-52, Scientific Services Division, NWS Central Region, Kansas City, MO, 29 pp.
Cold-air funnel clouds are a common sight along the Pacific Coast of the United States, particularly in the spring or fall.
On July 29, 2013, a cold-core funnel cloud touched down as an EF0 tornado in Ottawa, Ontario, Canada, causing extensive damage in the form of downed trees on a golf course. No advance weather watches or warnings were issued by Environment Canada, and the tornado was spawned from one of the few non-severe storm clouds moving through the area.
Other funnel clouds
Shear funnels might also occur with weak transient circulations at the cloud base of thunderstorms. Mesoanticyclones, which accompany , often exhibit these funnel clouds. Brief funnels also are observed with some rear flank downdrafts (RFDs) (within inflow or outflow areas, and especially within inflow-outflow interchange areas as RFDs interact with mesocyclones or flanking line updrafts) and streamwise vorticity currents (SVCs) feeding into mesocyclones.
Although not considered a separate kind of funnel cloud, some funnel clouds form with mesovortices associated with , which also can become tornadoes but are often not visible as funnel clouds or tornadoes because they usually occur within obscuring precipitation. Other "fair weather" funnel clouds include which are a very transient phenomena associated with extremely weak Horseshoe vortex.
See also
External links
- What is a tornado? (Charles A. Doswell III)
- Tornado Myths: A funnel cloud needs to touch the ground to be a tornado, OR the visible funnel is the tornado (Dan Robinson)
- USA Today article on small vortices
- TORRO Tornado FAQ
- Cold-core waterspouts over Lake Michigan in fall 2006
- news helicopter footage of cold-air funnel clouds in the St. Louis metropolitan area in spring 2007 (KTVI)
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