upon the systems of atmospheric circulation in the higher latitudes must be left, however interesting, in the hands of the theorist.

163. In the same way as many of the cyclones of the higher latitudes may be regarded as extensions of the great systems of barometric depression at the poles, so many of our anticyclones may be regarded as offshoots from the belts of tropical calm, which become detached to some extent from the latter, and are developed in a poleward direction. But But very many and important anticyclones have their birthplace in the higher latitudes, and possess systems of atmospheric circulation of their own, which appear independent of, and on their equatorial side are antagonistic to, the great currents which belong to the exterior of the tropical calm-belts. To a certain extent we can form some conclusion as to their formation at particular seasons of the year. For example, in the spring months of the northern hemisphere anticyclones are somewhat more common over the North Atlantic than at other times. It is at this season that over the more southern portions of the great continents of the northern hemisphere temperature is rapidly rising, while in the higher latitudes very low temperatures continue to exist at the earth's surface. The upward movement of atmosphere over the southern portions of the continents is thus accompanied with an increase of pressure in the direction of the poles; and it is to the anticyclones thus produced over the North Atlantic and over Northern Europe that the notorious character of the north-east surface winds in Western Europe,

which merit the title of the spring easterlies, is principally due. As our summer season approaches, insolation in the long days of the higher latitudes, by warming the floor of the atmosphere, raises the lower isobaric shells, and arrests the downward movement of the atmosphere over these regions. For the same reason, it is in the autumn of the northern hemisphere, when the temperature of the higher latitudes at the earth's surface is highest, that anticyclones over the North Atlantic and over Northern Europe are least numerous, and distinctly least permanent. The anticyclone is, however, a phenomenon which occurs at all seasons of the year in the higher latitudes.

164. These systems are, generally speaking, more characteristic of land than of ocean surfaces, for the very obvious reason that land surfaces have in winter a lower temperature than that of the ocean in their neighbourhood, and that the atmosphere on the ocean surface being more humid than that over the land, those extensive Inversion movements due to condensation of vapour which belong to the cyclone are much more prevalent over the oceans. Where these movements occur over land surfaces they are very frequently of the local type, which, while productive of violent showers and occasionally of tornadoes, are incapable of producing the larger cyclone.

165. As has been inferred, anticyclones are generally connected with a slow downward movement of the atmosphere, and the currents which flow out from them have not nearly the velocity of those rapid indraughts of air caused by a cyclone. The air in an

anticyclone being usually very dry, there is seldom a great quantity of cloud in an anticyclone, and the cloud-forms that do exist must be those which belong to slow horizontal or descending currents of the atmosphere.

166. In winter the anticyclone produces the sharpest frosts, owing to the absence of a vapour or cloud screen at night. A layer of Stratus Quietus in the daytime will effectually stop any of the feeble rays of the winter's sun from heating the cold earth below. This cloud is especially typical of the winter anticyclone, being a cloud of moderate Interfret, and generally requiring a downward motion of the atmosphere for its formation. As we shall see in the next chapter, it occurs everywhere where the atmosphere is settling quietly down, as in the interspaces between progressive cyclones, and as in the polar calms and areas of high pressure over large continents. It occurs, as already intimated, on the western edge of the typical cyclone shown in Plate VI.

Clouds of Radiation also are very common in anticyclones, especially at night time and in winter (§ 27). In the hot weather of the summer anticyclone, Stratus Quietus, Cumulo-rudimentum, small Cumulus, and occasionally Cumulo-stratus, simple Cirrus, and perhaps a little Cirro-macula, are often seen, although the two latter are generally left in rear of a progressive cyclone. Stratus Maculosus also often exists on the exterior of an anticyclone, but not far from the outskirts of an area of low pressure.

CHAPTER X

PREVALENT CLOUD-FORMS OF THE GLOBE

PRACTICAL SUGGESTIONS

167. It would be an almost impossible task to describe in detail the cloud-forms of the globe, because the manifold effects of very varied distribution of land and water, in conjunction with seasonable changes, cause innumerable complications even over a small area such as that of Western Europe. All that can be attempted in this chapter will be to indicate very roughly a few of the general characteristics of cloud distribution. Any further detailed descriptions would serve no useful purpose.

168. We have already seen that for general purposes the globe may be divided into two polar areas, two wide belts of relatively high pressure traversing it in the middle latitudes, and an equatorial belt of reduced pressure. The belts of high pressure are the sources of supply of two great systems of winds the Trades, which blow in a somewhat westerly direction towards the equatorial belt, and the anti-Trades, which blow in a somewhat easterly direction towards the polar areas. The whole system

has a periodic variation dependent on the inclination

of the earth's surface to the sun, and it is further extremely interrupted by the relative position of land. and sea surfaces during the different seasons. Where the equatorial belt crosses the two great oceans, the fluctuations of atmospheric pressure and the variations in the direction of the winds are comparatively unimportant, and it is here, and still more over the tradewind regions to the north and south of this zone, that the cloud-forms observed are most permanent. Where, however, the seasonal distribution of pressure between the tropics is considerably modified by the seasonal heating and cooling of land areas, causing the Trades to cross the line and to form the periodic winds called monsoons, great alterations in the character of the clouds necessarily result.

These

169. Within the belts of high pressure which lie outside the region of the Trades the characteristic forms of cloud vary considerably as we proceed towards the poles, and as we advance into the region of the anti-Trades we encounter those great nonperiodic fluctuations of pressure which are accompanied by non-periodic changes in the cloud-forms. changes are least marked as we advance into the interior of the great continents, and it is in the middle latitudes of the latter that the greatest seasonal variation exists, and in the least interrupted manner, owing to long periods of heat and cold, which are the effect of the continued action of solar and terrestrial radiation. The non-periodic disturbances of pressure extend far into the polar regions, and even, we infer, to the poles themselves; but in these regions the