CHAPTER VII

THEORY OF ATMOSPHERIC CURRENTS

It is,

118. It lies beyond the scope of the present work to deal in any but the most elementary way with those causes to which the complex atmospheric circulation prevailing over our earth is to be attributed. however, needful here to describe, as briefly as possible, the general theory of the movements of the atmosphere, partly because cloud-structure is everywhere correlated with these movements, and partly because, although over all parts of the globe clouds are susceptible of a single classification, yet particular species of cloud are not everywhere due to exactly similar causes, and do not therefore accompany or presage similar weather.

Much of what will be stated in the present chapter has been thoroughly investigated by other writers, to whom the author's apologies are due. If the reader desires to understand the full investigation of the general circulation of the atmosphere he must refer to Ferrel's Popular Treatise on the Winds.

119. It will be convenient, in the first place, to neglect the effects produced by the rotation of the earth on its axis. We imagine, therefore, a fixed

globe, having a belt of high temperature, maintained by the sun revolving round it, as in the geocentric theory of the ancient astronomers, and having also two poles of relatively low temperature. If we suppose that this sphere is without water, and that therefore the air enveloping it is devoid of watervapour, the superheated gases will expand over the thermal equator, and the chilled air will contract over the thermal poles. There will consequently be a head of air over the former region, and a meridional upper current will flow from this towards the latter regions, compensated by under currents from the poles towards the warm belt. There will be also a neutral surface between the two currents in which no horizontal movements will take place. Also the whole circulation would only take place in a downward and upward direction, and to and from the poles and the equator, were it not for the fact that diurnal heating and nocturnal cooling of the lower strata would give rise to slight displacements producing regular periodic westerly and easterly components of motion.

120. We will now consider the effects of the presence of water on this hypothetical sphere. In the first place, evaporation from water will lower the temperature of the surface from which it occurs, and of the air through which the water-vapour rises. Its next effect (§ 13) will be, in conjunction with dust particles, to render the temperature of the latter as well as that of the earth's surface more equable, by arresting the effects of solar and terrestrial radiation.

Its third effect will be observed in the condensation of vapour into cloud producing a rise of temperature in the surrounding air, and forming another screen for the strata of air below. Finally, precipitation, in the act of restoring to the earth some of the vapour previously withdrawn from it, will tend to decrease the temperature of the lower layers, and to check that state of unstable equilibrium (§ 5) which has preceded and accompanied condensation.

Looking now at these effects in their relation to the movements of the atmosphere, we see that they intensify the latter.

Condensation will obviously be greatest over the equatorial heat-belt, and this will tend still further to increase the head of air over this region, and therefore the velocities of the currents to and from the poles.

We have hitherto treated our hypothetical nonrotating sphere as being homogeneous. We will now consider the effect of placing two continents on this sphere. These continents may be regarded as circular, and each of about the size of Australia; one of these lies just to the north of the thermal equator, while the other is bisected by an arctic zone (Fig. 7). Owing to the different effect of solar radiation upon land and sea, the first-named continent will have an area on its southern side over which the temperature of the atmosphere will be higher than in the rest of the heat-belt. Also the arctic continent will have an area on its northern side colder than the remainder of the arctic zone, so that this area becomes the thermal

pole. The effect of the positions of the two continents is shown in the figure, where the dotted arrows represent the upper currents and the firm arrows the lower currents. The lower currents will flow from the north of the arctic continent to the south of the equatorial continent, and the upper currents in the opposite directions. Precipitation also will be greatest over the equatorial continent, especially over its mountainous districts.

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FIG. 7.-WITHOUT ROTATION.

121. We now come to the effects on atmospheric currents of the rotation of that sphere upon which they move. It was recognised more than 150 years ago that the Trade Winds, which flow from the northeast in the northern and from the south-east in the southern hemisphere into the equatorial calm-belt, owe their relative westward components of motion to the fact that the earth is rotating from west to east. Hadley was the first to explain the relative westward motions of the Trade Winds, but certain errors of his were not entirely cleared away till the year 1859,

when Ferrel deduced from Newton's law of the “Conservation of Areas" the true principle on which the relative motions of bodies over the earth's surface are to be explained. This principle may be thus simply stated "If a body moves in any direction upon the earth's surface, there is a deflecting force arising from the earth's rotation which deflects it to the right in the northern hemisphere, but to the left in the southern."

122. Examples of the laws on which this principle depends may help to make it clearer. A man walking quickly over a drawbridge which is itself revolving from left to right tends to fall of on the left-hand side. A rider whose horse swerves to the left tends to fall off on the right-hand side. Moreover, the amount of the defecting force which the earth exerts varies with the latitude, and is greater the nearer we approach the poles. It also, of course. varies with the velocity of the moving body. Thus. if a locomotive travel with a velocity of 45 miles per hour (20 m. per sec.) over a straight piece of line in latitude 45 in the northern hemisphere, the lateral pressure tending to throw it off the line to the right will be both part of the vertical pressure of the locomotive. But if in run on a straight line in latitude 60° this pressure will be th part of the weight of the engine.

When we come to the ease of a finid such as water we shall see that the surface of the water w not be level, but that the water will be higher on the

bs bank in the northern hemisphere than on the