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Figure 1.2 (a) A missile launched from the Equator has not only its northward firing velocity but also the same eastward velocity as the surface of the Earth at the Equator. The resultant velocity of the missile is therefore a combination of these two, as shown by the double arrow.

(b) The path taken by the missile in relation to the surface of the Earth. In time interval 7",, the missile has moved eastwards to Mh and the Earth to fi,; in the time interval T2, the missile has moved to M2, and the Earth to G2. Note that the apparent deflection attributed to the Coriolis force (the difference between M, and G1 and M2 and G2) increases with increasing latitude. The other blue curves show possible paths for missiles or any other bodies moving over the surface of the Earth without being strongly bound to it by friction.

axis of rotation axis of rotation

Figure 1.3 Diagram of a hypothetical cylindrical Earth, for use with Question 1.1,

QUESTION 1.1 Bearing in mind what you have just read, especially in connection with Figure 1.2, what can you say about the Coriolis force acting on a body mov ing above the cur\ cd surface i>l a hypothetical cylindrical Earth rotating about its ;i\is. as shown in Figure 1,3'.'

The example given above, of a missile fired northwards from the Equator, was chosen because of its simplicity. In fact, the rotation of the (spherical) Earth about its axis causes deflection of currents, winds and projectiles, irrespective of their initial direction (Figure 1.2(b)). Why this occurs will be explained in Chapter 4, but for now you need only be aware of the following important points:

1 The magnitude of the Coriolis force increases from zero at the Equator to a maximum at the poles.

2 The Coriolis force acts at right angles to the direction of motion, so as to cause deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

How these factors affect the direction of current flow in the oceans, and of winds in the atmosphere, is illustrated by the blue arrows in Figure 1.2(b).

When missile trajectories are determined, the effect of the Coriolis force is included in the calculations, but the allowance that has to be made for it is relatively small. This is because a missile travels at high speed and the amount that the Earth has 'turned beneath' it during its short period of travel is small. Winds and ocean currents, on the other hand, are relatively slow moving, and so are significantly affected by the Coriolis force. Consider, for example, a current flowing with a speed of 0.5 m s"1 (~ 1 knot) at about 45° of latitude. Water in the current will travel approximately 1800 metres in an hour, and during that hour the Coriolis force will have deflected it about 300 m from its original path (assuming that no other forces are acting to oppose it).

Deflection by the Coriolis force is sometimes said to be cum sole (pronounced 'cum so-lay'), or 'with the Sun'. This is because deflection occurs in the same direction as that in which the Sun appears to move across the sky - towards the right in the Northern Hemisphere and towards the left in the Southern Hemisphere.

The Coriolis force thus has the visible effect of deflecting ocean currents. It must also be considered in any study of ocean circulation for another, less obvious, reason. Although it is not a real force in the fixed framework of space, it is real enough from the point of view of anything moving in relation to the Earth. We can study, and make predictions about, currents and winds within which the Coriolis force is balanced by horizontal forces resulting from pressure gradients, and for which there is no deflection. Such flows are described as geostrophic (meaning 'turned by the Earth') and will be discussed in Chapters 2 and 3.

Because solar heating, directly and indirectly, is the fundamental cause of atmospheric and oceanic circulation, the second half of this introductory Chapter will be devoted to the radiation balance of planet Earth.