Waterspouts are tornadoes over water. But scientific work
over the last 30 or so years has led to a more complicated picture with
waterspouts differing in some ways from tornadoes over land, especially large
ones. Waterspouts and all the different kinds of tornadoes have a similar basic
structure with air moving upward. At the ground or ocean surface, winds are
rushing faster and faster as they swirl into the vortex and then upward. Often
with both tornadoes and waterspouts, the vortex is seen coming down from the
cloud, but not obviously touching the ground or ocean. Such vortices that don't
seem to touch the ground are called "funnels" or "funnel
clouds." It's important to say "seems" not to touch because
often, especially in the beginning, the vortex is invisible along part of its
path. We begin to see the vortex when its lower air pressure cools the air
enough to condense water vapor in the air into tiny water droplets.
How waterspouts form
1. The first sign, which can be seen from the air but usually
not from a boat, is the formation of a dark spot on the ocean. Smoke flares
dropped in these areas show the air is moving in a circle and upward. Many dark
spots die out without progressing any further. But some begin to take on a
spiral pattern of dark and lighter water.
2. At this second stage someone on a boat at the surface would
probably feel the wind shift and maybe increase. Also, if you looked upward,
you might see a funnel coming from a cloud overhead or off to one side.
3. People on boats will see the third stage. Even though it
might be invisible, a vortex is reaching the ocean surface from the cloud. When
the wind speeds reach around 40 mph, the wind begins to kick up spray in a circular
pattern — the spray vortex. At this time you might see the funnel pointing down
from the cloud toward the ring.
4. The fourth, or mature, stage 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.
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.
Where waterspouts are most likely
"Waterspouts probably
occur more frequently in the Florida Keys than anywhere in the world." Waters
around the Keys, especially from Marathon past Key West on westward to the Dry
Tortugas, probably see 400 or 500 waterspouts a year. Since they are so common,
most go unreported unless they cause damage. So many waterspouts hit the
Florida Keys because the weather and geography supply two necessary
ingredients. First, the islands and the shallow water along them help heat the
air. During the summer, waterspout season, the air is extremely humid with
temperatures in the mid-80s into the low 90s. The heat causes the air to rise.
As it rises, the air's humidity condenses into the tiny water droplets that
make up clouds. As water vapor condenses, it releases more heat that makes the
air rise even faster. Rising air currents are needed for waterspout formation. The
second important waterspout ingredient in the Keys seems to be the regular east
or northeast "trade winds" that blow right down the islands. These
winds help line up the clouds. Lines of clouds encourage waterspouts.
