838 Rain. 



prevail, however, the alternation of dry and wet seasons de- 

 pends on the winds. When the south-west monsoon is blowing 

 over India, for instance, there is no rain on the east coast, but 

 abundant rain on the west coast. During the north-east 

 monsoon these conditions are reversed. A little consideration 

 will show that all the above-mentioned seasonal variations 

 within the tropics depend on general laws already stated. 



Beyond the tropics there is less regularity. The fall of rain 

 depends on the prevalence of certain winds which bring 

 moisture with them, and these winds not blowing with any 

 regularity, the rainfall is similarly irregular. In countries 

 close to the tropics, there is a noteworthy dryness in summer; 

 for this reason clearly, that in summer the trades blow over these 

 regions, and bring with them " trade-wind weather." Further 

 north, however, though there may be a tendency to the preva- 

 lence of north-easterly winds in summer, this tendency is not 

 so marked as to produce a considerable defect of rain in the 

 summer as compared with the winter months.* 



In England we have one cause affecting the rainfall which 

 is worthy of special notice. I refer to the Gulf-stream. The 

 air above this warm stream is not only warmer than the sur- 

 rounding air, but is heavily laden with moisture. When the 

 western and south-western winds loaded with the vapour of 

 water begin to blow over England, they precipitate their 

 moisture in rain as they encounter the colder air over the land ; 

 but the manner in which this happens is variable with the 

 seasons, for in the winter months the moisture-laden winds 

 blow lower, and therefore precipitate their vapour earlier; 

 whereas in summer the clouds range higher, and therefore 

 travel farther inland before they fall in rain. The same effects 

 are observable in the Scandinavian peninsula, Norway receiving 

 more rain in winter than in summer; while Sweden, on the 

 eastern side of the Dovrenelds, receives more rain in summer 

 than in winter. 



Such are some of the general laws which affect the down- 

 fall of rain in various countries and at different seasons. There 

 is one circumstance involving the action of a yet grander law 

 — about which, however, considerable uncertainty still exists. 

 I refer to the difference observable between the northern and 

 the southern hemispheres. It has been already noted that 

 the mean position of the medial zone of calms and heavy 

 diurnal rainfalls lies some 4° or 5° to the north of the equator. 

 The total annual downfall of rain north of this medial line is 



* So far as my own observations oxtend, I should say that the two features of 

 our climate which may be most certainly depended on — which, be it noted, is not 

 saying much — are, heavy rains in July, generally in the last fortnight, and serene 

 weather during the second week of November. 



