Hurricanes get boost from ocean spray

A new mathematical model that describes airflow across the ocean’s surface suggests that droplets whipped from the tops of waves increase the speeds of winds well above what they’d be if the spray weren’t there.

Winds are caused by differences in atmospheric pressure between one spot on the map and another. Wind speeds typically are slower near Earth’s surface than they are at higher altitudes, a friction-based phenomenon called the boundary-layer effect.

Alexandre J. Chorin, a mathematician at the University of California, Berkeley, and his colleagues developed a model of flowing air masses that incorporates the effects of tiny suspended droplets such as ocean spray. That fine mist, whose droplets measure 20 micrometers or so across, dramatically reduces atmospheric turbulence, the team’s analyses suggest. Such turbulence is a major source of friction in the boundary layer.

In one of the team’s simulations that didn’t include ocean spray, wind speeds 20 meters above a wave-tossed sea measured a sluggish 4 meters per second (m/s) or so (equivalent to a 9-mile-per-hour breeze). But when the researchers included a layer of ocean spray 10 m above the surf in otherwise similar conditions, the higher-level winds increased to 30 m/s, which is just shy of hurricane gales. Chorin and his colleagues describe their findings in the Aug. 9 Proceedings of the National Academy of Sciences.

The effect of ocean spray on wind speed may explain why some sailors, even in ancient times, cast oil on the sea during stormy weather, says Chorin. The oil coated the surface of the water, reduced the formation of spray, and thereby calmed the wind around the ship, he suggests.

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