Waterspouts come in a variety of sizes and strengths, and arise from one of two different mechanisms. Dr Joseph Golden* defines the waterspout as a "funnel which contains an intense vortex, sometimes destructive, of small horizontal extent and which occurs over a body of water
." Waterspouts fall within the class of atmospheric phenomena known as convective vortices that includes tornadoes, dust-devils, landspouts, and hurricanes. These are circulations driven by, or associated with, either dry or moist convection.
Tornadic waterspouts are formed from mesocyclonic action, in a manner essentially identical to traditional land-based tornadoes. These water spouts can start as tornadoes over land., then move over a body of water. This type of waterspout can be fairly destructive.
The second type of waterspout is formed over water, initially due to warm temperatures in the lower atmosphere along with high humidity. These fair weather
waterspouts are generally not as dangerous, and are more common than tornadic waterspouts.
Waterspouts are most common over warm oceans, but they can occur over just about any body of water (I’ve seen them over Lake Superior’s frigid waters).
They commonly develop beneath lines of rapidly growing cumulus congestus clouds. These cloud lines can be quite shallow, averaging between 12,000 to 14,000 ft. deep with bases about 2000 ft. above the surface. The waterspout funnel forms in the rising air just ahead of any rainfall from these clouds in association with warm water temperatures and high humidity in the lowest ½ mile of the atmosphere.
While the thin-columned waterspouts appear to be sucking water up from the water surface, what is actually seen is the condensation
of water vapour in the rotating vortex air. As the column rotates, the highly humid air is cooled by expansion to its condensation
point. When the water vapour in the vortex condenses, it makes the whirling mass visible. At the surface, the vortex winds stir the water into mushroom-shaped water sprays at the funnel base.
* Much of the significant early research on waterspouts comes from the investigations of Dr Joseph Golden in the waters surrounding the Florida Keys during the 1960s and 1970s.
Golden distinguishes five distinct stages in a waterspout's life cycle:
1. Dark spot: A prominent circular, light-coloured disk first appears on the surface of the water, surrounded by a larger dark area of indeterminate shape and diffuse edges. This light inner circle measures approximately 7.5 to 31 metres (25 to 100 feet) in diameter.
2. Spiral pattern: Next, a pattern of light- and dark-coloured surface bands spirals out from the dark spot on the water surface. This stage represents the primary growth phase of a waterspout in which the vortex intensifies while simultaneously expanding its radius of influence.
3. Spray ring: A dense swirling ring (annulus) of sea spray, called a cascade, appears around the dark spot with what appears to be an eye. Stage 3 marks the point at which convergence of inward-flowing angular momentum reaches a critical value in the ring surrounding the dark spot. This accumulation of angular momentum is accompanied by a gradual increase in wind
speeds. When the winds attain a speed of 83 km/h (52 mph), a spray ring arises, and the lower portion of the waterspout begins to advance.
4. Mature vortex: The waterspout, now visibly extending from the water surface toward the overhead cloud mass, achieves maximum organization and intensity. The mature funnel often appears hollow with a surrounding shell of turbulent condensation. The spray vortex may rise to a hundred metres (several hundred feet) or more and often creates a visible wake and associated wave train as it advances. In Stage 4, the rotation and the maximum winds around the centre of the spray vortex peak, and the funnel grows to its maximum diameter and length. Forward motion also reaches its maximum during the latter portions of Stage 4.
5. Decay: The funnel and spray vortex begin to dissipate when the inflow of warm air into the vortex weakens. The spiral pattern begins to disappear during the end of Stage 4 and by the beginning of Stage 5 is completely gone. The waterspout's forward motion decelerates as well. During Stage 5, the funnel will attain its maximum tilt from the vertical. Finally, when the flow of upward lifted warm air is disrupted by strong sinking motions of rain cooled air, the waterspout integrity starts to collapse. Dissipation occurs abruptly when an advancing shower
overtakes the waterspout from the rear.