146 The Realm of Nature CHAP. 



the north of England and south of Scotland it is April, in 

 the Scottish highlands it is May, and in Orkney it is June. 

 The average distribution of climate shown in the maps, 

 although correct on the whole, cannot be depended upon to 

 hold good at any special place for any particular month. 

 Such maps are of great value in choosing a place to reside 

 in, but of very little use for planning a pleasure trip. The 

 conditions of weather are somewhat complicated, but appear 

 to depend mainly on the distribution of atmospheric pressure, 

 which may be classified into certain well-marked types. 



205. Anticyclones. An anticyclone is a portion of the 

 atmosphere in which the pressure is highest at the centre, 

 and diminishes nearly uniformly in all directions. The 

 wind in an anticyclone blows spirally outward, as is illus- 

 trated in the high-pressure regions shown in the Isobaric 

 maps. In the northern hemisphere the circulation of surface 

 wind round the edge of an anticyclone is in the same 

 direction as the hands of a watch move, in the southern hemi- 

 sphere in the opposite direction, as explained by FerrePs or 

 Buys Ballot's Law. An anticyclone when once formed is 

 a very steady arrangement of pressure, and usually lasts 

 for many days or even weeks at a time. This being so, 

 it is evident that a supply of air must be continuously 

 renewed from above to take the place of that passing out 

 as surface winds. Air in fact passes through an anti- 

 cyclone much as grain does through a pair of mill-stones, 

 though of course without suffering any physical change. 

 In the upper regions of the atmosphere air must be moving 

 inward and sinking downward to maintain the anticyclone, 

 and the pressure in the upper region of the atmosphere 

 must thus be least above the spot where it is greatest on 

 the Earth's surface. This deduction has been proved to 

 be true by observations at mountain meteorological stations. 

 The surface winds of an anticyclone are usually light and 

 variable. As the air is descending from above, it contains 

 very little water- vapour, and no clouds are formed. Hence 

 in summer, anticyclonic weather is brilliant, hot, and calm, 

 with haze at night or heavy deposits of dew, on account of 

 great cooling by radiation. In winter an anticyclone is 



