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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



THE RELATION OF METEOROLOGY TO HORTICULTURE. 

 By R. H. Curtis, F.R.Met.Soc. 



[Lecture given on April 10, 1907.] 



Rainfall. 



In a previous lecture on the relation of Meteorology to Horticulture which 

 I gave last year (Journ. R. Hort. Soc. vol. xxxii. p. 104) I dealt chiefly 

 with the subject of temperature, but in explaining the use of the wet-bulb 

 thermometer I then referred at some length to the fact that water is 

 a large and most important constituent of the atmosphere, in which it is 

 always present in the form of an invisible gas or vapour, although the 

 quantity varies greatly in different parts of the world, and changes also 

 in the same region almost from hour to hour with variations of the 

 temperature of the air and of other conditions. 



I showed that in certain circumstances, which almost invariably 

 involve the cooling of the air, this vapour may be condensed, and that it 

 then becomes visible as a cloud or fog consisting of numberless particles 

 of water each of extreme minuteness — so minute indeed as to be quite 

 invisible individually. 



To understand the formation of these minute particles of water we 

 must regard the invisible steam from which they are derived as being 

 itself composed of still more minute atoms, each of which is separate and 

 distinct from its neighbours. When the air becomes sufficiently cooled 

 to produce condensation these molecules of steam coalesce with those 

 immediately adjoining, forming themselves into groups, or minute drops 

 of water, which occupy in their new form very much the same position as 

 they had previously filled as atoms of gas. There is, however, another 

 constituent of the atmosphere which plays a very important part in the 

 formation of these water particles — namely, the excessively minute atoms 

 of dust which are everywhere present to a greater or less degree, and 

 which exert a very material influence upon other phenomena besides the 

 formation of rain-drops. These dust particles form nuclei upon which 

 the vapour molecules condense, each atom of dust forming the foundation 

 of a separate mass of water, so that, speaking generally, we may say that, 

 other conditions being favourable, the greater the dustiness of the air the 

 more numerous will be the particles of water formed, and the greater will 

 be the density of the resulting fog or cloud. 



A good deal of thought has been expended in trying to explain how 

 it is that these particles of water, forming large clouds, float in the air 

 instead of at once falling to the ground, because water, being heavier than 

 air, is as little able to floiit in the atmosphere as iron is able to float in 

 water. An explanation which used to be very generally accepted was that 

 t he drops were hollow, like a soap bubble, and therefore floated because of 

 their relatively large displacement. But if the centre of the drop be a 



