Waterspouts:
More Than Just a Curiosity to Serious Sailors
By Andy Johnson
Retired Meteorologist/Hurricane Hunter
(Previous owner: Cal-20, Cal-25, Cal-9.2)
Figure 1. Early developmental stage of a waterspout with very faint
filament-like vortex structure and small area of spray at surface
(From:
Austrailian Severe Weather)Imagine sailing in the early morning, a moderate land breeze in your face, fresh smell of sea air, a sturdy boat performing at her best, reaching freely across the waves between the shore and a stationary line of scattered towering cumulus clouds further offshore. You know that this line of clouds is formed by a gentle land breeze front, which is propagating offshore as the air, cooling over the land areas, becomes heavier, sinks and slides seaward under the relatively warmer air over the sea surface. Your "no sweat" morning weather forecast was for partly cloudy skies, and a 30 percent chance of an afternoon shower. You are relaxed and mesmerized by the gentle sound of the hull slipping through the water as a brilliant morning sun is giving birth to a new day.
Suddenly, your 10-year old son asks, "Hey Dad, what is that line hanging down from that cloud over there?" He points in the direction of the line of cumulus and you are startled by the sight of a small funnel shaped appendage embedded in the cloud base about a mile from your position. The "line" your son is referring to is a very faint filament-like structure, barely visible, which extends from the cloud downward about halfway to the surface. At the surface, you can barely discern what appears as a little ball of spray, apparently unconnected to either the filament or the funnel.
You are now snapped out of your trance and you realize you are face-to-face with a developing waterspout. What do you do?
This happened to me. My son, Craig, now a Cal List member, was the vigilant observer. We were sailing our Cal-20 back to her homeport after a hauling out for annual maintenance. The spout was structured very similar to that in Figure 1. We watched with great curiosity as it whimsically meandered across the Mississippi Sound dissipating before reaching shore. We were totally helpless as this 2000-foot long "dragon snake" slowly passed us bye.
Being a meteorologist, I was aware of the dangers but this and a few other close encounters with waterspouts while sailing caused me to take them very seriously. It is my hope that the information I provide here will help other sailors escape the wrath of the "dragon snake that rises out of the sea."
This article describes the stages of development of waterspouts, where they most often form, comparisons with tornadoes, and realistic safety measures if you encounter one in your boat. .
Not a Tornado Over Water
In the past, waterspouts were described as tornadoes over water. But recent scientific research has provided us with a more complete but more complex picture. Waterspouts and tornadoes have similar basic structure with air spiraling upward. At the ground or ocean surface, winds accelerate as they swirl into a vertically oriented vortex. With both tornadoes and waterspouts, the vortex is only seen when the pressure inside drops enough to condense water vapor from the moist air producing tiny water droplets. In both systems, damaging wind actually occurs at the surface before the visible portion (funnel cloud) of the vortex reaches the ground.
Figure 2. Waterspouts form in areas of low-level convergence where swirls of colliding air become vertically extended and intensified by rapidly growing clouds.
Waterspouts form primarily from low-level convergence of unstable air. This convergence results in swirls near the surface, which become more intense as they are stretched vertically by growing clouds. They begin near the surface and grow upward. Tornadoes form from a "meso-cyclone," large rotating thunderstorm, with roots in the middle levels of the atmosphere and grow downward to the surface.
While I was in graduate school, I had the privilege of meeting Dr. Joseph Golden, now a senior scientist with the National Oceanic and Atmospheric Administration (NOAA), as he did his pioneering research on waterspouts while pursuing his PhD at Florida State University. Dr. Golden flew into 16 different weak to moderate waterspouts, one of them 26 times, and determined structure and wind speed ranges. He speculated, "a significant fraction of the so-called Bermuda Triangle incidents are probably from waterspouts," and defined five life cycle stages.
Life-Cycle Stages of Waterspouts
Stage 1. The first sign of a developing waterspout is the formation of a dark circular area on the sea surface, similar to a "cat’s paw" but smaller and usually moving very slowly. This spot is approximately 10-20 yards in diameter, caused by slightly increased wind. Smoke flares dropped in these areas show the air is moving in a circle and upward. Many of these "dark spots" die out without developing any further.
Stage 2. Some "dark spots" begin to take on a spiral pattern of dark and lighter water. At this stage someone on a boat at the surface would probably only feel a wind shift in direction and some increase in speed. A small "funnel" may be visible embedded in the base of the cloud overhead.
Stage 3. Even though it might be invisible, a continuous vortex extends from the cloud base to the ocean surface (see Fig. 1). As the wind increases to around 35 knots, sea spray becomes visible in a circular pattern around the surface vortex. A funnel is usually seen at this stage extending at least part way down from the cloud base toward the center of the surface ring of spray.
Stage 4. The fourth, or mature stage (see Fig. 3) is when the funnel reaches all the way from the cloud to the ocean. You can usually see through the funnel - it's really a thin cloud of tiny water droplets. During this stage, small waves are being kicked up and the spout leaves a bubbly wake behind as it moves across the ocean. These tiny bubbles could be carbon dioxide and other gases that are dissolved in the water that are caused to effervesce by the low air pressure in the spout's center - like a bottle of soda that's just opened.
Stage 5. In the fifth, and final stage, the spray vortex weakens and the funnel becomes shorter and maybe more tapered. It often twists around and the bottom of the waterspout may move out from under the cloud. The waterspout's dissipation usually occurs when rain begins falling from the parent cloud. Cool air brought down by the rain cuts off the supply of warm, humid air.
Waterspout Breeding Grounds
Dr. Golden states that, "Waterspouts probably occur more frequently in the Florida Keys than anywhere in the world." This area, especially from Marathon Key to Key West and westward to the Dry Tortugas, probably experiences between 400-500 waterspouts per year. Most go unreported unless they cause damage. Their prevalence here is because the weather and geography supply two necessary ingredients.
First, the very warm shallow water heats the air and keeps it moist. During the summer, waterspout season, the air is extremely humid with temperatures in the mid-80s into the low 90s. As the air heats irregularly, warmer pockets become buoyant and rise. As these buoyant bubbles of warm moist air rise, the air's high humidity causes quick condensation and generates cumulus clouds. As water vapor condenses, it releases another source heat, latent heat, which makes the air rise even faster. Rising air currents are needed for waterspout formation.
Figure 3. Mature intense waterspout photographed from aircraft. Note swirls of dark and light-colored water characteristic of the mature stage.
(From NOAA Photo Library at http://photolib.noaa.gov/)
The second important waterspout ingredient is the orientation of the Keys. They line up parallel to the prevailing northeast trade winds. Increased surface friction and heating along the islands chain causes surface convergence. This convergence generates swirls, which become stretched vertically by the rapid growing cumulus clouds. This vertical stretching causes an increase in angular momentum increasing the intensity of rotation resulting in the intense vortices we call waterspouts. Clouds tops that spawn waterspouts in the Keys generally range in altitude from 18,000 to 22,000 feet.
After the Florida Keys, the next most active U.S. waterspout area is the southeast Florida Coast from around Stuart southward to Homestead. Tampa Bay has the greatest number of damaging waterspouts, probably because the shores of the Bay are densely populated.
Other areas where waterspouts are common include the following. The northern coast of the Gulf of Mexico along with the Atlantic Coast northward to Chesapeake Bay are also likely to see waterspouts. Waterspouts have been reported on the West Coast from Tatoosh Island, Washington, southward to San Diego, but they tend to be weak and short lived. Waterspouts are also seen occasionally in the Great Lakes and in Utah's Great Salt Lake.
Survival in a Boat
As with tornadoes, because of their small scale, precise prediction of waterspout occurrence is not possible so real-time detection becomes critical. Detection by Doppler radar, which only sees precipitation, is possible only for large well-developed spouts and tornadoes. Therefore, visual detection is your best friend. Become cognizant of your weather situation before you sail and if you are in a tropical air mass and you observe towering cumulus clouds, note their direction of movement and keep an eye open upstream from your position. Remember that waterspouts tend to come from clouds with a dark, flat bottom when there is just the first hint of rain.
If one heads your way, try to escape by going at right angles to its path. And if you think it is going to hit you, douse all sails and secure all hatches. You can survive most garden variety spouts this way.
Summary
To summarize, waterspouts are not really tornadoes but have similar structure. They form from weak to moderate cloud systems and build upward. They occur most often in the Florida Keys but also appear less frequently in many other locations. They are menacing to boats. When sailing in tropical air masses, stay informed via marine weather broadcasts. Early visual detection is critical and the best escape heading is perpendicular to its path.
References