Earth Sciences / 57On the surface of a rotating sphere, the speed at which any point moves depends on its latitude,
because that determines the distance it must travel: a point at the equator travels faster than one
at a higher latitude. Air at the equator is moving eastward at the same speed as the surface
beneath it. If it moves away from the equator, because it is not attached to the surface, it continues
to travel eastward at the same speed (slowing steadily by friction), which is now faster than the
surface beneath it, so its motion has an eastward component in relation to the surface. Similarly,
air moving towards the equator is moving eastward more slowly than the surface and so it
appears to drift westward, in fact because the surface is overtaking it. This is the Coriolis effect
and it accounts for the fact that air does not move north or south along straight paths in relation
to the surface. The strength of the Coriolis effect increases with distance from the equator and
in 1865 the American meteorologist William Ferrel (1817–91) pointed out that in low latitudes
the conservation of angular momentum would be more influential than the Coriolis effect (BARRY
AND CHORLEY, 1982, p. 138).
What Hadley appreciated was that air, warmed at the equator, will rise, cool as it rises, and descend
again. This establishes a convective cell of moving air which drifts to the east as it moves away from
the equator and to the west as it returns. We identify winds by the direction from which they blow
(i.e. the opposite of the direction in which they blow), so this accounts for the easterly trade winds
experienced at the surface in the tropics and also proposes westerly winds at high altitude. This
tropical cell, in reality a system of several cells, is known as the ‘Hadley cell’.
Rising equatorial air produces a region of predominantly low surface atmospheric pressure. The
air cools near the tropopause and descends in the subtropics, where surface pressure is
predominantly high. Very cold, dense air also sinks over the poles, forming polar high-pressure
regions. Air flows out from these regions at low level and rises again in middle latitudes, forming
a second set of cells. These two drive a third, mid-latitude system of cells, comprising air flowing
away from the equator at low level, rising where it meets air flowing in the opposite direction
from the pole, and dividing so that some of its air feeds the polar cell and some returns
equatorward, descending in the subtropics with the descending air from the Hadley cell. Figure
2.18 illustrates this flow and the winds associated with it. The intertropical convergence zone
(ITCZ) is where the two tradewind systems meet. Air rises gently, producing a low-pressure
belt near the surface, and this in turn often results in calm air, the region sailors called the
‘doldrums’, a name it acquired in the middle of the last century.
Where the cells meet, near the tropopause in the subtropics and again at about 60°, there is a sharp
difference in temperature in the air to either side of a boundary. These temperature gradients
produce strong westerly air flows, known as the ‘jet streams’. The subtropical jet stream is fairly
constant. The polar front jet stream, at an altitude of 9–15 km, is much more irregular, varying in
its latitudinal location and sometimes disappearing altogether, but it also produces the strongest
winds. These can reach 150–250 km h-1 and more than 450 km h-1 in winter, when the temperature
gradient is strongest.
Atmospheric convection cells produce the boundaries (fronts) between bodies of air at markedly
different temperatures which give rise to the jet streams, but it is the jet streams that dominate the
weather conditions experienced below. This is especially true of the polar front jet stream, which
develops waves and then breaks into cells. This process most commonly occurs in February and
March in the northern hemisphere, each complete cycle lasting several weeks. Figure 2.19 illustrates
four stages in what is known as the ‘index cycle’. At first (1), the winds are zonal (i.e. they flow
steadily from west to east) and there is little mixing of the air to either side. Waves start to
develop (2) as the jet stream widens and its velocity increases. Air is now flowing more to the
north and south and so it is influenced by the Coriolis effect and the conservation of angular