Monitoring the weather

Pressure (continued)

Air movement along 'isobars'
Why does wind blow around isobars rather than directly from high to low pressure? Imagine you are looking down from above one of the poles of the earth as if it were a merry-go-round, with you at the centre at point A. If you threw a ball directly to a friend on the rim at point B you would miss, because by the time the ball reached the rim the rotation of the merry-go-round would have taken your friend to point C.

To people on the merry-go-round the ball would appear to have been deflected to the left and to have followed a curved path. To people not on the merry-go-round the ball would appear to have followed the straight path.

The principle is the same on the earth. Moving objects will appear to have been deflected to the left in the southern hemisphere and to the right in the northern hemisphere. The deflections are imperceptible for objects like footballs travelling short distances, but they are important over long distances. Corrections have to be made for artillery shells, for example, or they will not hit their target. Pilots must also make navigational corrections to their flight plans.

The atmosphere spins with the earth as though it were a solid body. If it didn't there would be extremely strong winds, particularly at the equator, where a point on the earth is moving at 1670 km/hr because of the earth's rotation.

Any movement of the air relative to the earth is the wind. If a parcel of air at point X in the southern hemisphere moves towards the equator, it is moving to regions where the movement of the earth is greater and, to a person on the earth, the parcel of air will appear to be moving more slowly, and so have been deflected westward. Conversely, if the parcel of air moves toward the pole, it is moving to areas where the movement of the earth to the east is slower, and so the air is apparently deflected eastwards, once again to the left of its direction of motion. In the northern hemisphere, the deflections are to the right.

This is known as the Coriolis effect (after the French mathematician who explained it) and the 'apparent' force that causes the 'apparent' deflections is the Coriolis force. It increases with increasing latitude and wind speed, and alters the direction of the wind, but not its speed.

The Coriolis force can therefore balance the pressure force so that, in the southern hemisphere, the air will flow clockwise around a centre of low pressure and anticlockwise around a centre of high pressure. When the Coriolis force and the pressure force are in balance, the wind blows along the isobars and not across them. This is called the 'geostrophic wind'.

The geostrophic wind is always an approximation to the actual wind, because in the real world there are other forces at work. But when it is a good approximation it is extremely useful. It is a reasonable approximation in middle latitudes, and a good approximation in high latitudes. It is a poor approximation in the tropics where the Coriolis force is weak.

Foucault's Pendulum

In 1851 a French physicist, Leon Foucault, demonstrated the effect of the earth's rotation in classical fashion by suspending an iron ball about 25cm in diameter on a wire 61 metres long inside the dome of the Pantheon in Paris.

Beneath it he built a circular ring on which he placed a ring of sand. A pin at the base of the ball scraped the sand each time it swung pendulum-fashion some two metres across the ring.
With time the pendulum swing changed in a clockwise direction, cutting out a swathe of sand.

After 24 hours the pendulum moved approximately 270°. Had the experiment been done at the equator there would have been no rotation; at the north pole, it would have been a full 360°. In the southern hemisphere the rotation would have been anticlockwise.

This classical experiment demonstrated the rotation of the earth and the apparent force which results from it, first identified by a French mathematician, Gustave Gaspard de Coriolis, in 1835.