Waterspouts are often thought of as tornadoes over water, but that is only a partial explanation of what waterspouts are. They do sometimes form from tornadoes that move out over water from land. Sometimes they form over water beneath supercell thunderstorm clouds. These are tornadic waterspouts; they are destructive but not common. More common are fair-weather waterspouts, forming over water beneath vertical cumulus or cumulonimbus clouds. They are made up of rising columns of rotating air, at first invisible, then becoming sheer curtains of spinning water droplets. According to the National Weather Service, most fair-weather waterspouts last from two to 20 minutes and can move at 10 to15 knots (11 to 17 mph), sometimes faster. At maturity, fair-weather waterspouts can briefly generate hurricane-force winds.
To form, waterspouts need warm, humid air, as well as the cooler air provided by a cloud system. As a patch of warm air on the surface of the water rises, the cooler air around it begins to rotate. Because humid air over the water forms a copious amount of water vapor, and because this wet air is actually lighter than dry air, the water vapor swirls up into the air, cooling and condensing, becoming a "cascade" around the vortex. As the water vapor condenses into droplets, it releases more heat, increasing the vortex effect as it rises skyward. After a short time, the waterspout dissipates as the air within it cools. Waterspouts most often form during the warm season in tropical and subtropical waters. Dr. Joseph Golden, a waterspout expert with the National Oceanic and Atmospheric Administration (NOAA), says that the Florida Keys is where waterspouts occur more frequently than anywhere else in the world, though they have also occurred at higher latitudes, such as the Great Lakes and the Grand Banks off the coast of Newfoundland.
A Waterspout's Life Cycle
Dr. Joseph Golden has identified five stages of waterspout formation. Waterspouts are first evidenced by a raised dark spot on the surface of the water, usually only visible from the air. Though invisible, this indicates contact of a complete funnel of air between the cloud system and the water's surface. Then a spiral pattern of light- and dark-colored bands of water develops. When wind speed reaches around 40 mph, the spray vortex, or cascade, is formed, often rising several hundred feet into the air. Next, the fully visible waterspout reaches maturity and is seen reaching all the way from the ocean to the cloud. The final stage is decay of the waterspout, usually occurring as warm air feeding the waterspout dissipates, often cooled by rain showers developing from the cloud system.
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