Aug 22 2012
Though these look a lot like tornadoes they’re actually waterspouts, a phenomenon that fascinates me because I rarely see it.
Waterspouts don’t occur in Pittsburgh because they require lots of open water and just the right weather conditions. The best place to see them is in the Florida Keys but you don’t have to go that far at this time of year. They also form on the Great Lakes in late summer and early fall.
It’s possible to have a tornado over water, and yes it’s called a waterspout, but those are rare and dangerous. Tornadic waterspouts spin down from above but the really cool and much more common fair weather waterspouts spin up from the water to join the clouds. These require warm water, light winds, and humid air between the water and clouds. They go through five stages as described on this NOAA webpage:
- Dark spot: A light-colored circle appears on the water’s surface surrounded by a dark area.
- Spiral pattern: The dark spot spins and forms a spiral on the water around it.
- Spray ring: The spinning makes water spray up around the dark spot. The spray forms a small “eye” like the eye of a hurricane.
- Mature vortex: The spray ring gets organized and moves up to join the cloud. Now it looks like a waterspout. Sometimes you can see through its hollow center.
- Decay: The funnel and spray vortex dissipate as warm water stops feeding them. The waterspout disappears.
The frequency of waterspout sightings on the Great Lakes has increased since NOAA began tracking them in 1957. There was a big outbreak of them on all five lakes September 27 to October 3 in 2003.
To learn more about waterspouts watch this dramatic video on the NOAA website.
(photo from NOAA by L. Glover. Click on the image to see the original)