The General Circulation of the Atmosphere

THE GENERAL CIRCULATION OF THE ATMOSPHERE The Distribution of Temperature over the Earth.—The distribution of temperature over the earth is usually represented by two charts, one of the mean temperature for January and one of the mean temperature for July. These two months rep resent the extremes of winter and summer, the other months being intermediate between them. (See any physical atlas.) We shall consider first the chart for January. It will be seen that in the southern hemisphere the isotherms, or lines of equal temperature, run across the chart in a regular manner, indicating approximate symmetry of distribution of temperature about the poles, except that over South America, South Africa and Aus tralia, the isotherms sweep downward over the land, indicating that the land is warmer than the sea in the same latitude. In the northern hemisphere there are two centres of extreme cold, one in northeast Siberia and one over Greenland. If in middle latitudes we restrict our attention to any one parallel of latitude, we note that the temperature is higher over the western coast of a conti nent than over the eastern coast of the same continent. This dif ference is associated with the fact that prevailing westerly winds bring with them the conditions of their region of origin.

The chart for July shows again in the southern hemisphere an approximately symmetrical distribution of temperature about the South Pole. In the northern hemisphere there are centres of high temperature over North Africa, over the southern por tion of North America, and to the north of India, but the run of the isotherms is irregular. The land is, however, distinctly warmer than the sea, the difference being especially well marked over the north Pacific ocean and the contiguous continents.

The Distribution of Surface Pressure over the Earth.— Charts giving the distribution of pressure show in the southern hemisphere a belt of high pressure encircling the earth about lati tude 3o°, with somewhat higher pressures over the oceans than over the continents. These belts are known as the "sub-tropical anticyclones" or the "sub-tropical high pressure belts." Over the northern hemisphere the extensive land masses complicate the phenomena. The January chart shows the sub-tropical anticy clonic belts over the north Atlantic and north Pacific oceans, but in addition to these there is an intense anticyclone centred over Asia, and another over North America. Over the northern portion of the North Pacific, and also just south of Greenland, there are centres of low pressure.

In July, the anticyclonic centres over the north Atlantic and north Pacific persist, and extend somewhat further north. A

shallow centre of low pressure is situated over north-east Canada, and a much deeper centre of low pressure is centred to the north west of India, extending over the whole of Asia, and even over a part of north-east Africa. The subtropical anticyclonic belt is no longer traceable over southern Asia.

The Distribution of Winds over the Globe.

There is a simple relation between the direction of the wind and the distri bution of pressure, which is known as Buys Ballot's law, after the Dutch meteorologist who first enunciated it. It may be stated as follows : "In the northern hemisphere an observer who stands with his back to the wind will have lower pressure to his left than to his right. In the southern hemisphere the reverse holds." In terms of isobars on a chart, this amounts to saying that the wind tends to blow round the isobars keeping low pressure to the left in the northern hemisphere, and to the right in the southern hemisphere. In practice the wind at the surface is found to blow slightly across the isobars into low pressure, at an angle of 2o° to 3o°.

Keeping this law in mind, we can interpret the pressure charts for January and July in terms of the prevailing winds. We shall first consider the chart for January. The equator is marked by a shallow belt of low pressure, to each side of which the pressure increases with distance to-yards the pole. The region on the equatorial side of the subtropical anticyclonic belts is therefore marked by easterly winds having a component towards the equator. These winds known as the trade winds are north-easterly in the northern hemisphere and south-easterly in the southern hemisphere. Between them is the equatorial belt of low pressure, known as the doldrums, with calms or light variable winds. It is situated slightly north of the equator in the northern winter, and moves slightly further north in the northern summer. The centres of the sub-tropical anticyclonic belts are regions of light winds. On the poleward sides of the anticyclonic belts the winds are westerly, especially in the southern hemisphere, where there is only slight disturbance by land masses. Over the north Atlantic the prevailing winds are westerly. The circulation over Asia is clockwise round the centre of high pressure, and over the China seas the winds are north-easterly. - The regions of prevailing west erly winds on the poleward sides of the sub-tropical anticyclones are not regions of steady winds. Here the depressions of middle latitudes produce intermittent variations of conditions, yielding considerable local variations of wind, temperature and rainfall.