2019-07-01_Discover

JULY/AUGUST 2019. DISCOVER 73

What’s

in a name?

A tropical cyclone is

the official name for these

swirling systems of storms, but

you’ll hear different names based

on their location; specifically, they’re

hurricanes in the U.S. and typhoons

in countries in the western Pacific.

Other places, like Australia and

India, just call them tropical

cyclones.

Hurricane

season typically

extends from

June through November,

though storms have struck

both earlier and later

than that. Just when you

thought it was safe to

move back near the

water...

The Curious Coriolis Effect

Even standing still at the equator, your body’s actually

moving at about 1,000 mph, thanks to Earth’s rotation.

But that speed changes depending on where you

are. The surface — and you,

standing on it — spins

more slowly the farther

from the equator you

get. But winds aren’t

tethered to the

surface like we are.

So, as a gust of

wind moves away

from the equator, it

begins to outpace

the ground beneath

it because the gust

maintains its original

speed while the Earth

slows down beneath it.

As a result, it bends. This

means the wind sucked

toward the center of a tropical

depression or hurricane bends to its right in the

Northern Hemisphere, and to its left in the Southern

Hemisphere. The tension between this motion and

the pull of the low-pressure system at the center of

a storm creates a vortex that spins counterclockwise

in the Northern Hemisphere and vice versa in the

Southern.

This phenomenon — or the lack of it — also explains

why there’s a hurricane-free zone within 5 degrees of

the equator. Coriolis forces, named for French scientist

Gaspard-Gustave de Coriolis, aren’t strong enough to

form a vortex there.

Wind shear is a difference in wind speed over a

short distance. A lot of wind shear — say, if winds

are slow near the surface but fast up in the clouds

— tears apart rising air. When this occurs over the

ocean, it prevents the formation of low-pressure

systems that spawn hurricanes.

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Cameras on the

International Space

Station captured a

view of Hurricane

Florence the morning

of September 12, 2018,

as it churned across

the Atlantic.

The pressure gradient

sucks winds in (blue)

while Coriolis forces (red)

bend them, resulting

in the characteristic

hurricane spin (black).