Coriolis Effect

Large-scale winds (such as the monsoons— Section 12.1) are deflected by the Coriolis effect, just as ocean currents are (Section 11.4). Therefore, air flowing from a high-pressure region to one of low pressure is turned to the left in the southern hemisphere, until the pressure-gradient force to the right exactly matches the Coriolis force (Figure 12.8). This balance of forces is known as the geostrophic balance, and the resulting wind is the geostrophic wind (Note 12.C). The adjective 'geostrophic' means 'Earth turning', the cause of the Coriolis effect.

The geostrophic wind blows along an isobar, not directly from a place of high pressure to one of low, but at right-angles to the pressure gradient. This fact, that winds blow along isobars, may seem surprising, as if water flowed along a contour line round a hill, instead of down it. The important point is that the geostrophic wind in the southern hemisphere blows clockwise around a low and counterclockwise around a high, at a speed which is proportional to the spacing between the isobars (Note 12.C). The situation was summed

Figure 12.7 Mean sea-level pressures in July in the southern hemisphere.

Figure 12.7 Mean sea-level pressures in July in the southern hemisphere.

up by Christoph Buys-Ballot (1817-90) in his 'Rule' (1857)—pressure is Low on your Left when you face the wind in the southern hemisphere.

Wind measured about 1,000 m above the ground is close to geostrophic. In fact, the similarity between (i) arrows or streamlines showing the wind's direction, and (ii) tangents to the isobars, is so good that forecasters customarily plot the isobars alone, for places away from the equator. Near the equator, the Coriolis force is too weak to balance any pressure-gradient force (i.e. the winds are non-geostrophic), so air there does flow directly down the pressure gradient, tending to equalise adjacent highs and lows. The consequently flattened pressure gradients are the reason for the weak winds around the ITCZ (Figure 12.5). Forecasters use streamlines to show such winds.

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