69 
9,.—RaInFALL. 
By W. Marriort, Esq. 
(Read February 17th, 1902). 
Mr. W. Marriort said he hoped his audience would not be dis- 
appointed, as he had brought no paper to read, nor had he come 
prepared to lecture, but rather to give a conversational address 
on the subject of rainfall. The subject was probably a familiar 
one to the members of the Society, as the Meteorological Com- 
mittee of the Society had done good work for many years past 
by issuing a very useful Monthly Return of Rainfall in the 
district south of London, compiled from daily records. 
Whence, however, comes the rain? If ice be put into a glass 
of water and its temperature reduced below that of the surround- 
ing atmosphere, the outside of the glass becomes wet. This is 
caused by condensation of the moisture in the atmosphere owing 
to the temperature of the glass being below dew point, whereby 
the air is induced to part with a portion of its moisture. The 
atmosphere contains moisture, and the higher the temperature 
the more moisture it can absorb. On reaching colder currents of 
air in the higher regions the moisture is condensed and forms 
clouds; and when the temperature is reduced below the dew 
point the moisture is parted with in the form of rain. 
The prevailing rain-bearing wind in the British Isles is the 
south-west, which comes off the Atlantic warm and highly 
charged with moisture. It strikes against the hills on the west 
and rain is discharged. Thus the heaviest rains occur chiefly on 
the west coast, and the amount increases according to altitude. 
Mr. Marriott said that he had collated the rainfall data for the 
Croydon district, published by the Meteorological Committee, for 
the ten years 1891-1900, according to height above sea level, and 
found that the rainfall increased with altitude, in the same way 
as in other parts of the country. The following were the 
results :-— 
Altitude. Rainfall. 
100 feet. 22°08 inches. 
200 ,, 23°41 _~—C,, 
300 ,, 24°54, 
400 ,, 26:87 is, 
500 ,, 27°18 si, 
600 ,, ———_— 
700 ,, PAST Vii eree 
Peculiarities in the position of localities will modify the apparent . 
fall. Thus a gauge placed at the top of a hill near the sea may 
not record so great a fall as one placed a short distance inland 
