Why tornadoes often occur after a hurricane lands on land

Tornadoes start at the outer edges of hurricanes. When a storm hits land, an increase in surface friction can lead to air vortices, explains New York Times.

Photo: Shutterstock

As Hurricane Isaias moved through the mid-Atlantic states on Tuesday, August 4, despite its winds steadily weakening, a new danger emerged: tornadoes.

“Hurricanes often produce tornadoes, and they are similar to those produced by severe thunderstorms in the Central Plains,” said Jana Houser, assistant professor of meteorology at Ohio University.

When tornadoes form, they form in the outer rain bands of hurricanes that contain convective cells called thunderstorms.

But as long as the hurricane is moving over water, tornadoes won't form, Houser explains. The thing is that the surface of the water is relatively smooth, and in the formation of tornadoes, “the biggest reason is surface friction.”

Once the rain streaks reach the ground, surface friction increases significantly. This slows down storm winds near the ground.

“You suddenly create a situation where the wind speed and (quite often) the wind direction changes,” she said. This is called wind shear and can cause rotational movement in the air.

On the subject: Tropical storm Isaiah sweeps east coast: at least 6 dead

This first creates a rotating cylinder of air parallel to the surface. But, as with any thunderstorm, the hurricane's convective cells create strong updrafts. They can tilt the rotating air vertically: this is how a tornado is born.

If the updraft is very strong, the rotating air will be more compacted, with a smaller diameter. When this happens, the tornado can intensify, much like a figure skater extending her arms to increase her spin speed.

This can lead to a powerful destructive tornado.

“But they are typically not as long-lived and tend to be a little weaker than those formed from supercells in the Central Plains,” she said.

The threat of a tornado diminishes as the hurricane weakens.

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