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A rare waterspout formed off the coast of San Francisco over the weekend. Waterspouts are tornadoes that touch on water.

The "tornado on water" that touched down off Ocean Beach was captured on video March 18, which helped meteorologists confirm the event.

Doppler radar can see rains and winds associated with a storm but can't confirm that a storm touched the ground. But in the YouTube video, water splashes from the ocean under the funnel cloud, confirming touchdown, said Mark Stobin, a meteorologist with the National Weather Service in Monterey, Calif.

In areas where intense funnel clouds commonly form over water the Florida Keys and the Adriatic Sea are two examples waterspouts will frequently form along a line of developing thunderstorms.

The same storm in the Bay Area also brought another unusual weather event an EF-1 tornado. The twister destroyed a metal building in Santa Rosa, according to the Storm Prediction Center in Normal, Okla.

A tornado's strength is based on the amount of damage it causes. The Enhanced Fujita tornado damage scale runs from 0 (minor damage) to 5 (powerful enough to destroy a house). An EF-1 tornado has wind speeds of 86 to 100 mph (138 to 177 kph).

Most of the Earth's tornadoes touch down in the area of the United States known as Tornado Alley, bordered by the Dakotas to the north, the Gulf Coast to the south, the Rocky Mountains to the west and the Appalachian Mountains to the east. Southeast of Tornado Alley is Dixie Alley, home to the deadliest tornadoes.

In these areas, warm, moist Gulf of Mexico air collides with cool, northern air in the spring. But tornadoes can occur elsewhere and at other times of the year. There can even be winter tornadoes.

California, which has different weather patterns than those in Tornado Alley, averages about five tornadoes per year, according to the National Climatic Data Center.

"It's not so much like you have over in Montana or the Plains states," Stobin told OurAmazingPlanet. "It's more of a cold front in itself."

Instability along the cold front and channelized winds generate swirling storms on the West Coast, Stobin said.

Email OurAmazingPlanet staff writer Brett Israel at bisrael@techmedianetwork.com. Follow him on Twitter @btisrael.