Waterspouts are not just “tornadoes over water.” Meteorologists admit they still have much to learn about these phenomena, but there is a typical “waterspout cloud” that usually generates them. These dark, flat-bottomed cumulus clouds generally get no taller than 20,000 feet. As clouds go, this is noticeably low. Waterspouts get their energy from heat in the water, so they are most frequently seen in warm waters when winds are light. They are most prevalent in the Florida Keys from May through September, averaging 500 per year there, but they also occur in the Caribbean and in the Atlantic hundreds of miles off the U.S. East Coast.
Waterspouts vary greatly in size and shape, and they move in a relatively consistent direction at speeds from 2 to 80 mph. Favorable conditions for waterspout formation can persist for days, and it is not unusual to see several at a time. Some dissipate after a few minutes, others last for hours.
Some sailors erroneously believe that waterspouts are harmless. Packing winds up to 200 mph, they have, however, capsized many boats and have swept the crews of others overboard. Their formation can’t be predicted, nor can they be detected by radar, which means weather services cannot issue warnings. Once a spout is spotted, however, warnings are usually relayed via VHF radio by boaters and weather stations.
If you see a waterspout, even at a great distance, you should start securing items on deck. Drop and tightly furl your sails. Place all loose items down below. Keep a lookout for other spouts that may form closer to you. Any crew who must remain on deck should be secured to the boat in a harness. You cannot count on being able to outrun a waterspout, but their paths are generally straight. You should try to follow a course that is 90 degrees to the spout’s path to increase the chance of it missing you. If you hear a hissing or a roar, everyone aboard should go below.