The Science Book

126

See also: George Hadley 80 ■ Robert FitzRoy 150–55

A

ir and ocean currents do

not flow in straight lines.

As the currents move,

they are deflected to the right in

the northern hemisphere, and

to the left in the southern. In the

1830s, French scientist Gaspard-

Gustave de Coriolis discovered the

principle behind this effect, now

known as the Coriolis effect.

Deflected by rotation

Coriolis got his ideas from

studying turning waterwheels,

but meteorologists later realized

that the ideas apply to the way

winds and ocean currents move.

Coriolis showed how, when an

object is moving across a rotating

surface, its momentum seems to

carry it on a curved path. Imagine

throwing a ball out from the center

of a spinning merry-go-round. The

ball appears to curve around—even

though to anyone watching from

outside the merry-go-round it is

actually moving in a straight line.

Winds on the rotating Earth are

deflected in the same way. Without

the Coriolis effect, winds would

simply blow straight from high

pressure areas to low pressure

areas. The wind direction is in fact

a balance between the pull of low

pressure and the Coriolis deflection.

This is mostly why winds circle

counterclockwise into low pressure

zones in the northern hemisphere,

and clockwise in the southern

hemisphere. Similarly, ocean

surface currents circulate in

giant loops or gyres, clockwise

in the northern hemisphere and

counterclockwise in the south. ■

WINDS NEVER

BLOW IN A

STRAIGHT LINE

GASPARD-GUSTAVE DE CORIOLIS (1792–1843)

IN CONTEXT

BRANCH

Meteorology

BEFORE

1684 Isaac Newton introduces

the idea of centripetal force,

stating that any motion in a

curved path must be the result

of a force acting on it.

1735 George Hadley suggests

that trade winds blow toward

the equator because Earth’s

rotation deflects air currents.

AFTER

1851 Léon Foucault shows

how the swing of a pendulum

is deflected by Earth’s rotation.

1856 US meteorologist William

Ferrel shows that winds blow

parallel to isobars—lines that

connect points of equal

atmospheric pressure.

1857 Dutch meteorologist

Christophorus Buys Ballot

formulates a rule stating that

if the wind is blowing on your

back, an area of low pressure

is to your left.

Earth’s rotation causes winds to be

deflected to the right in the northern

hemisphere and left in the southern.

Initial direction

Initial direction

Deflected

right

Deflected left