780 MR J. Y. BUCHANAN ON THE 



In the months from June to October it rains at least two days out of every three; the 

 tension of aqueous vapour is high, and the atmosphere is generally nearly saturated 

 with it ; this is the weather which accompanies the strong south-west wind known as 

 the Monsoon. 



If we take the meteorological record of a year at any such place we find that the 

 sorting out of different kinds of weather has been done by Nature, and the discussion 

 has necessarily reference to the different kinds of weather, even although this may not 

 always find expression. At the Poles, where the year consists of one long day and one 

 long night, we cannot doubt that the meteorology must be characterised by much sim- 

 plicity. In Europe, we have dry seasons and rainy seasons ; but, whereas, in India the 

 dry season occupies completely one-half of the year and the rainy season occupies com- 

 pletely the other half without any mutual interference, our wet and dry seasons alternate 

 as many times in the course of a single day. Here it would seem to be indicated that 

 we should do for ourselves what Nature does for us in the Tropics, and sort out the 

 weather before discussing it. 



The prime factor in determining the climate of a place is its Latitude, because this 

 determines the altitude to which the sun rises from day to day and the length of time 

 that it is above the horizon each day. The heating power of the sun on any horizontal 

 portion of the earth's surface varies with the sine of its altitude at the moment. All 

 the elements of climate and weather depend ultimately on this factor, and its variability 

 produces a corresponding variability in the weather. It is only necessary to consult a 

 table of sines to see where the greatest variability is likely to occur. Thus, at the 

 equator the sine of the sun's meridian altitude varies between 0*924 and 1*000, at 

 either of the Tropics, between 0*684, and 1*000 in lat. 45°, between 0*367 and 0*930, and 

 in the latitude of Ben Nevis (56° 48' N.) it varies between 0*169 and 0*835. At the 

 equator the sun's heating power at noon only varies by 7|- per cent, of its maximum 

 amount, while at Ben Nevis the variation is 80 per cent. 



But the latitude of a place determines not only the intensity of the sun's heat which 

 it receives, it also determines the intensity of the cooling to which it is exposed by 

 radiation into the upper regions of the atmosphere and into space. This goes on all 

 day independently of the sun's heating, but it becomes more apparent after the sun 

 has set, and produces the greater effect the longer is the duration of the night, and 

 this is a function of the latitude. While the altitude which the sun attains measures 

 the heat which it supplies, and the length of the night determines the amount which is 

 lost, both the heat received and that lost at any particular place may be in excess or in 

 default of what it is entitled to, owing to its latitude alone. This is due to secondary 

 actions set up by the primary heating and cooling, whereby one place may receive, in 

 addition to its own, a supply of heat or of cold to which it is not entitled, thereby 

 altering its climate as well as that of the other place which has supplied the heat or cold. 



The principal secondary agencies through which the sun works are the atmosphere, 

 in its motion, both horizontal and vertical, and in its changes of volume ; and the sub- 



