Tornadoes are more commonly associated with areas less vulnerable to coastal storms, like the southern and midwestern U.S. But they can form once a hurricane transitions from sea to land and its winds encounter greater friction.
Between 1994 and 2014, a full two out of three tornadoes that hit the U.S. between August and September had a link to tropical cyclones, according to The Weather Channel.
While hurricanes are over the ocean, “the water is kind of acting like a frictionless surface [and] winds at the ground don’t get a whole lot slower as they’re encountering the water,” said Jana Houser, an associate professor at Ohio State University who specializes in radar analysis of tornadoes.
“Water does not have much friction,” noted William Gallus, a professor of meteorology at Iowa State University, who compared a hurricane over the ocean to a can of soup being spun, spinning as a single unit in one direction.
But once a storm approaches land, Houser said, the winds closer to the ground begin to slow as they encounter resistance from the ground, while those at higher altitudes accelerate.
This alone doesn’t form a tornado, as the winds are still spinning horizontally, Houser said, but when a strong thunderstorm is added to the mix, it reorients the winds from “spinning like a bike tire to spinning like a top.”
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