weather and waves

Waterspouts

By Jack Williams

Imagine you’ve been diving from a boat in the Florida Keys on a humid summer day with hardly a breath of air stirring, when suddenly someone points off in the distance and yells, “Look at that.” When you look, you see a solid-looking but almost transparent funnel stretching from the bottom of a cloud to the water, where it’s kicking up spray.

It reminds you of the videos you’ve seen on television news of tornadoes on their way to destroy Midwestern towns, except the funnel, the cloud and even the sky don’t seem to be as dark as in the news videos of tornadoes. You’re fascinated, but also fearful and curious. What’s going on? What causes a twirling rope of wind to stretch from a cloud to the ocean? You know tornadoes are deadly, and this looks like a tornado, but it doesn’t look all that frightening. You are most likely seeing a waterspout, which is not merely a “tornado over water” as dictionaries or out-of-date books about weather will tell you.
 

What Is a Waterspout?

A waterspout, like a tornado, consists of air that’s rising into a cloud. Various natural forces that researchers are just learning to understand in detail have given the rising air its spinning motion. As the air rises, more air rushes in to replace it, and begins spinning and rising into the cloud.
And yes, waterspouts can be dangerous. In 1980 one overturned a shrimp boat near Corpus Christi, Texas. They are nothing to play with, as some boaters do when they zoom into the edges of waterspouts. Most of the time their winds aren’t as fast as those of large tornadoes. Winds of most waterspouts rarely top 50 mph/81 kph. But you can’t count on a waterspout being harmless; some have been observed with winds as fast as 200 mph/322 kph. Powerful tornadoes, like those that ripped across Oklahoma and Kansas on May 3, 1999, destroying swaths of well-built houses, are rare along the Gulf of Mexico and Florida coasts, and are unknown over the warm water around the world’s tropics. Such places do produce waterspouts, however.
Waterspouts and tornadoes are cousins, and the dividing line between them isn’t always clear-cut, even to research meteorologists. Most waterspouts range from 10 to 300 feet/3 to 91 m in diameter and last 10 to 15 minutes. A large Plains tornado, in contrast, might be as much as a mile wide, last for more than an hour and produce winds of 300 mph/483 kph or faster.
In the last 20 years or so meteorologists have stopped talking about waterspouts being tornadoes over water, as researchers have discovered many basic differences between large tornadoes and waterspouts. To show how complicated weather can be, researchers have also discovered that some small tornadoes that form over the Great Plains have more in common with Florida Keys waterspouts than with strong tornadoes. Some researchers use the term “landspouts” for these twisters, but other meteorologists wince when they hear that term.
No matter what they’re called, all these vortices have one thing in common: They are attached to clouds. This separates them from “dust devils,” the whirling winds usually seen over warm land that carry dust and dirt into the air, but fizzle out in a clear sky, sometimes hundreds of feet above the ground.

Tornadoes vs. Waterspouts

What are the differences between big tornadoes and waterspouts? Tornadoes, especially the killers with winds of 150 mph/242 kph or more, are a part of powerful, long-lasting thunderstorms known as supercells that can tower more than 40,000 feet/12,191 m above the ground. These storms — and the tornadoes they produce — are powered by the heat released when humidity in rising air condenses to form clouds. But winds blowing from different directions at various altitudes help supply energy, as well as the spinning motion that helps create tornadoes.
Waterspouts, in contrast, are attached to cumulus clouds that normally top out at 18,000 or 20,000 feet/5,486 or 6,096 m. Sometimes a waterspout’s parent cloud might produce a few lightning bolts, but often there’s no lightning. The heat energy of water vapor condensing to form the cloud supplies most of the energy that powers waterspouts. They don’t have the energy of high-altitude winds to draw on as strong tornadoes do.
Large, extremely violent tornadoes are most common in the Great Plains, in the Midwest and across inland parts of the Southeast. They usually spare the coastal Southeast, but a few violent tornadoes have hit central Florida, including coastal areas. These are most likely in the spring.
In the United States, waterspouts are more common in the summer, with the Florida Keys having more than any other location. Researchers have estimated that as many as 400 to 500 waterspouts form in the Keys during an average year, but most go unreported because they are so common. Waterspouts are also relatively common along Florida’s Atlantic Coast, and along the Gulf Coast from Florida to Texas. While they are less common, waterspouts have also been reported along the West Coast and on all of the Great Lakes. Waterspouts are sometimes reported in the tropics, but good statistics on them aren’t available.

Forecasting

Unlike waterspouts, strong tornadoes are the product of widespread, vigorous weather systems that forecasters usually see coming a day or two ahead of time. The National Weather Service begins issuing alerts well before tornadoes begin forming, and television weathercasters make sure their viewers know dangerous weather is on the way.
Once thunderstorms begin growing, forecasters use Doppler weather radar, which can detect wind motions in storms, to look for indications that a tornado is forming. Often they can spot the very earliest stages of a potential killer tornado and issue warnings even before the funnel touches the ground. The larger the tornado, the easier it is to detect by radar.
Since much smaller and less violent weather systems produce waterspouts, the signs that something is brewing are harder to spot. While National Weather Service offices in all U.S. coastal areas have Doppler radar, it isn’t likely to detect early signs that a waterspout or a series of waterspouts is forming, because the air motions involved are too small for the radar to see.
But meteorologists at National Weather Service offices where waterspouts are common, such as at Miami and Melbourne, Florida, are researching them, looking for ways to improve forecasts. As a result, weather offices will often produce a “hazardous weather outlook” to alert boaters to the possibility of waterspouts several hours before any form. When reports of waterspouts are received, Weather Service offices issue special marine warnings giving their locations. The Weather Service urges boaters who see a waterspout to report it quickly, because this helps forecasters issue timely alerts.
 

How Waterspouts Form

Waterspouts generally form with clouds that have flat bottoms, and often just as light rain is beginning to fall from the cloud. Sometimes you might see a funnel that doesn’t seem to reach all the way to the water hanging from the cloud. Researchers in airplanes and helicopters often see a circular, light-colored disk on the water under the funnel, which is surrounded by a large, dark area. But someone in a boat isn’t likely to see this.
This so-called “dark spot” is a good sign that the vortex does reach from the cloud to the water; it just hasn’t become visible yet. The dark spot then might develop into a pattern of light- and dark-colored bands on the water that spiral out from the dark spot. This is a sign that a stronger swirling pattern is developing in the funnel, much of which is still invisible. As the waterspout grows stronger, it begins kicking up a ring of sea spray around the dark spot.
As the wind swirling around the funnel grows to around 40 mph/64 kph, it begins to carry the spray upward in a circular pattern known as a spray vortex. The funnel begins to become more and more visible as the low air pressure inside it cools the air enough for water vapor to begin condensing into tiny droplets. The funnel that you see is really a swirling cloud.
Soon, the funnel and spray vortex begins falling apart as the flow of warm air into the funnel begins to weaken. Often, rain that begins falling nearby brings down cool air that will strangle the waterspout by cutting off its supply of warm, humid air.
While waterspouts don’t have the deadly records of property destroyed and lives lost that large tornadoes do, they can be dangerous and should be avoided. The best advice when you spot one is to try to get out of its way by going at right angles to its path. Even a weak waterspout could send loose objects flying around a boat to become deadly missiles.
If you see one waterspout, you should keep an eye out for more, since they often form in groups. They seem to be most likely when winds are generally calm at the surface. Since weather conditions change slowly in the summer over the warm waters where waterspouts are most common, it is likely that if you see one waterspout, you might see others the following day.