PRECIPITATION 29 



Nairn, the fall on igth October was 4 inches in twenty-four hours, 

 while the rainfall for the whole year was 43-28 inches, so that 

 more than 9 per cent, of the total fell in twenty-four hours. 



On the same day heavy rain fell in the upper valleys of the 

 Spey and the Findhorn. The result was very extensive flooding, 

 notably at Kingussie, and a considerable interference with traffic. 

 The season being relatively advanced and the district one of high 

 mean elevation the precipitation on the higher hills took the form 

 of snow, but the fall on the low ground, as already stated, was 

 sufficient to swell the rivers to flood level. Such floods, produced 

 in this way, are not very uncommon, and whether they occur in 

 the local district or are merely recorded in the newspapers, should 

 be used as the starting-point for a lesson on torrential rainfall. If 

 4 inches of rain in twenty-four hours causes flooding, what must 

 we expect of countries where, as in the United States, falls of 

 5-7 inches in twenty-four hours are not infrequent ? At Bombay 

 24 inches have fallen in one night. All books of reference give 

 examples of similar heavy falls. The point is not to emphasise 

 the actual figure, but to correlate torrential falls with floods, to 

 contrast the frequency of such flooding in certain countries with 

 its relative rarity on any considerable scale here. 



If no examples of considerable flooding are available the fall 

 in a summer thunderstorm should be measured. One or two 

 figures may perhaps be useful as a basis of comparison. Thus on 

 2nd August 1906, it is estimated that about 5 inches of rain fell in 

 three hours at Moel Hebog in North Wales. In 1887, 2-24 inches 

 fell in forty minutes at Lednathie, Forfarshire. Much heavier falls 

 certainly occur in what are called " cloud bursts " in the western 

 states of North America. 



Even more important, however, than the actual amount of 

 rain which falls at any one spot, is the area over which the torrential 

 fall is distributed, for this has of course a great influence on the 

 amount of flooding produced. Thus during a historic flood at 

 Pennsylvania in 1889, it was estimated that a fall of 8 inches 

 occurred during three days over an area of 12,000 square miles, with 

 a smaller fall over a larger area. In England, on 28th and 29th June 

 1906, i| inches of rain fell over the very large area of 24,000 square 

 miles, comprising nearly the whole of the southern part of the 



