17th April 2006

Waterspouts

Source: Excerpt from The Book " Weather "

Distribution: Most frequent over coastal areas in middle latitudes. Height: Typically 1500 feet but can be as high as 3000 feet. Cause: Tornado over water, or rotation of air mass over water. Associated Weather: Showers from associated cloud. Hazard Warning: Can be a hazard to shipping. Waterspouts are rapidly rotating columns of air that form over lakes and oceans. They resemble tornadoes over water, and in some cases, are exactly that. However, they generally do not require severe thunderstorms to generate and sustain their motion, and are more often associated with congestus clouds. Waterspouts are classified as either tornadic or non-tornadic. Tornadic waterspouts are formed by the same mechanisms as tornadoes on land, and are relatively rare. Less intense, non-tornadic waterspouts appear to be caused by a pre-existing rotation near the surface of the water, combined with some form of updraft. This produces a funnel of rotating air that extends from the water to the base of the cloud. Non-tornadic waterspouts are most common in late summer or early fall. At this time the combination of warm sea-surface temperatures and cold air currents produces instability and strong updrafts. People often assume that a waterspout draws water up from the sea or lake below. In fact, apart from a small area of spray at the base of the spout, the water in the funnel is a result of condensation caused by very low pressure within the spiraling air mass. Waterspouts can occur in isolation or in clusters. Sometimes, the first indication of a developing waterspout is a shadow on the water where the rotating air is disturbing the surface. Once fully formed, a waterspout will tend to move slowly along a curved path for about 15 minutes until cooler air gradually enters the funnel, causing the spout to rapidly dissipate.

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Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.