The Science Book

80

B

y 1700, it was known that

persistent surface winds, or

“trade winds,” blow from a

northeasterly direction between

a latitude of 30°N and the equator

at 0°. Galileo had suggested that

Earth’s eastward rotation made it

“get ahead” of the air in the tropics,

so the winds come from the east.

Later, English astronomer Edmond

Halley realized that the Sun’s heat,

at its greatest over the equator,

causes air to rise, and that rising

air is replaced by winds blowing

in from higher latitudes.

In 1735, English physicist

George Hadley published his

theory on trade winds. He agreed

that the Sun causes air to rise, but

rising air near the equator would

only cause winds to flow toward it

from the north and south, not from

the east. As the air rotates with

Earth, air moving from 30° N

toward the equator would have its

own momentum toward the east.

However, Earth’s surface moves

faster at the equator than at higher

latitudes, so the surface speed

becomes greater than the air’s

speed and the winds appear to

come from an increasingly easterly

direction as they near the equator.

Hadley’s idea was a step on

the way to understanding wind

patterns, but contained errors.

The key to the deflection of wind

direction is in fact that the wind’s

angular momentum (causing it to

rotate) is conserved, not its linear

(straight-line) momentum. ■

WINDS, AS THEY COME

NEARER THE EQUATOR,

BECOME MORE

EASTERLY

GEORGE HADLEY (1685–1768)

IN CONTEXT

BRANCH

Meteorology

BEFORE

1616 Galileo Galilei points to

trade winds as evidence of

Earth’s rotation.

1686 Edmond Halley proposes

that the Sun traveling west

through the sky causes air to

rise and be replaced by winds

from the east.

AFTER

1793 John Dalton publishes

Meteorological Observations

and Essays, which supports

Hadley’s theory.

1835 De Coriolis builds on

Hadley’s ideas, describing a

“compound centrifugal force”

that deflects the wind.

1856 American meteorologist

William Ferrel identifies a

circulation cell in the mid-

latitudes (30–60°) where air

pulled into a low-pressure

center creates the prevailing

westerly winds.

Polar easterlies

Mid-

latitude

westerlies

Easterly

trade

winds

60°N

Wind patterns result from Earth’s

rotation combined with circulation

“cells” as hot air rises, cools, and falls

in polar cells (shown in gray), Ferrel

cells (blue), and Hadley cells (pink).

60°S

30°S

30°N

0°

Earth rotates

toward the east

See also: Galileo Galilei 42–43 ■ John Dalton 112–13 ■

Gaspard-Gustave de Coriolis 126 ■ Robert FitzRoy 150–55