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What Is a Tornado That Occurs Over Water?

Sometimes, on dark afternoons when the winds rise and the sea becomes turbulent with froth crusted waves, some unlucky mariners can glimpse a funnel of spinning water and wind illuminated by the lighting of a vigorous storm. It is a tornadic waterspout, which is a tornado that has touched down on the water. Like other tornadoes, it is fierce and destructive. Fortunately, it is often short lived.

Description
- A tornadic waterspout is a tornado that forms over the water or one that forms on land and then travels over the water. Unlike a fair-weather waterspout, it develops during a severe thunderstorm and may be accompanied by hail, lighting and wind speeds of up to 318 miles per hour. It differs from hurricanes or typhoons, which are also rotating storms, in that it requires a large difference in the wind's speed, direction and temperature at different altitudes, while hurricanes require none of these conditions. They are also usually less than a mile wide and last less than two hours. A hurricane is much larger and lasts for days.
Formation
- When moist, tropical air meets a cold front, it begins to rise. If it is significantly warmer than the rest of the atmosphere, it continues to rise instead of cooling and falling again, creating a thunderstorm. If a thunderstorm produces strong winds that are significantly different in temperature, speed or direction at different altitudes, a rotating column forms, usually turning clockwise parallel to the ground. This mesocyclone is the beginning of a tornadic waterspout.
Life Cycle
- When the mesocyclone touches down on the water, it forms the tornadic waterspout. At first, the abundance of warm, moist air feeds it, causing it to grow. But gradually, the downdraft behind it becomes an area of cool air, which surrounds the tornado and causes it to lose momentum. After a few minutes, the funnel contracts and disappears.
Waterspout Threat Level
- The National Weather Service creates waterspout hazard maps for designated coastal areas. These maps predict the likelihood of a waterspout through a color-coded threat level system that ranges from non-threatening to extremely threatening. When the threat level is extreme, there is a 36 percent chance or greater that a waterspout will form within 12 miles of that location. If you sight one, move as quickly as you can at a 90 degree angle to its apparent path. Even the mildest tornadic waterspout can cause significant damage to your craft and crew.

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