CaribbeanSan FranciscoAcapulcoHow winds work
Portuguese
passage to India
Using the south Atlantic
gyre, from the late 15th
century Portuguese
navigators – beginning with
Vasco da Gama in 1497 –
pioneered the route around
the southern cape of Africa
and east to India and the
Spice Islands (now Maluku,
Indonesia). In the process,
they also stumbled on the
coast of what’s now Brazil.Warm air around the Earth’s equator rises
before rolling over through high altitude and
descending back to the ground at around 30°
latitude in both the northern and southern
hemispheres. These air masses then return
to the equator along the surface as winds to
complete this huge vertical loop. These great
rolling currents in the atmosphere, called the
Hadley cells (named for English meteorolo-
gist George Hadley), are convection currents
that turn in opposite directions, like paired
rollers straddling the equator.
As these surface winds return towards
the equator they are deflected to the west
by the Coriolis effect – a consequence of
the fact that the Earth is rotating – and this
creates a band of prevailing winds blowing
towards the west: the easterly trade winds.
Beyond the Hadley cells are two more
great rolling circulation currents: the
Ferrel cells, named for a 19th-century
US meteorologist. These circulate in the
opposite direction, so the surface winds
they produce blow in the opposite direction
to the trade winds – these are the westerlies.
Discovery of these bands of winds and
associated ocean currents – pictured on
the globe below and on the map to the right –
allowed European explorers, traders and
colonisers to expand their ambitions.Spanish Manila galleons
Having discovered the passage around the tip of South
America to the Pacific in the early 16th century, Spanish
navigators recognised gyres corresponding with the
familiar circular currents in the Atlantic. This powered
trade between Spanish bases in the Philippines and Mex-
ico, with Chinese goods such as silk, porcelain and spices
heading east in exchange for silver from the Americas.2232Trade winds and exploration1Polar cellPolar cellHadley cellHadley cellFerrel cellFerrel cellDoldrumsNorth-east trade windsWesterliesWesterliesSouth-east trade windsИЗП
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