GILLIGAX : EFFECTS OF STORM ON BARDEX FELL. 389" 
A grp.phic description of what is called a " Gill- brack " is 
given in a letter quoted in the Geological Survey Memoir on 
the country around Ingleborough, i). 91. This occurred in 
Dentdale on January 28th, 1752, and was due to the heavy 
rain following a fall of snow, and so causing the snow to slide 
bodily down the mountain side. 
Ilkley A\as the scene of a disastrous storm on July 12th, 
1900, particulars of which have been supplied me by Mr. Albert 
Wilson, F.R.Met.Soc, of Ilkley. The rainfall on this occasion 
was as much as 5-40 inches, and as the rain gauge was somewhat 
shaded, probably 6 inches would represent the correct amount. 
Much damage was done in the town, liouses being thrown down 
and propert}^ carried awa.y. The Wharf e rose six to seven feet, 
but did not overflow its banks, as it was so Ioav prior to tlie 
storm.* 
Probable Cause of Such Storms. 
Storms such as I have described are well knowu in all moun- 
tainous districts, from which numerous valleys radiate, and 
the reason assigned for them is the following : — The air whicli 
has been in contact with the earth in the valley during the sunny 
hours of the morning gets warmed, and, rising along the slopes, 
carries up the water vapour of the lower strata of the atmosphere^ 
so that the relative humidity of the air on top of the mountain 
increases in the afternoon, while it decreases in tlie valleys below. 
The dynamical cooling which takes place condenses the water 
vapour into clouds, and when the air is damp enough they en- 
velop the mountain tops, and not infrequently develop into 
thunderstorms. 
Dr. Julius llamit says : " There is a tendency to afternoon 
ruins among micuntains, and during the warmer months to 
thunderstorms. Such rains occur even when the general weather 
conditions do not indicate any precipitation, and when the 
general lowlands are having the most beautiful weather. These 
Symoa'ri British Rainfall, 1900, p. Ki. 
f Handbook of Climatology. 
