187 
1911-12.] The Sun as a Fog Producer. 
there was any sunshine — the limit in one case being only after sunning 
compared with what it was before. It will be noticed from the table that 
the air was often thick before sunrise with the wind from the pure direc- 
tion, but the history of these cases shows that the wind had previously 
been from an impure direction, and the sun would appear to have worked 
up all its impurities, since further sunshine had no effect on it, as may be 
seen from the table. 
With regard to the barometrical conditions under which these fogs are 
formed, I find that nearly all the cases entered in the table took place in 
an ticy clonic conditions. But it is doubtful if this has any real significance, 
further than that it is generally under these conditions that the air circula- 
tion becomes slow enough to allow of the accumulation of impurities. In 
cyclonic areas the circulation is usually quick enough to keep the air fairly 
pure. The position of the centre of the anticyclone with regard to the 
position of the place of observation does not seem to be of any import- 
ance, as these fogs formed with the centre either to the north, south, east, 
or west. The densest of the fogs, however, took place when there was no 
general circulation, but light airs moving in different directions at different 
parts of the . country, a condition eminently suitable for the accumulation 
of impurities in the atmosphere. 
Summing up the results of these observations, it has been found : — First , 
that when the wind is slight and brings moist air from an impure direction, 
sunshine invariably destroys the clearness of the air and causes a thick 
haze or a fog. Second , that air from pure directions is not thickened by 
sunshine.* Third, that when there is no sunshine there is no thickening 
of the air though the air be damp and from an impure direction. Fourth, 
that high winds, by preventing the accumulation of impurities, tend to check 
the formation of haze by sunshine, and clear away sun-formed haze. Fifth, 
that when there is more than three degrees difference between the dry- and 
wet-bulb thermometers the sun only causes a slight increase in haze. 
It seems probable that the so-called summer or heat haze may also be 
due to the action of sunshine on the impurities in the atmosphere. Thunder 
* An instance of the effect of local impurities on the formation of these fogs was seen on 
23rd January of this year at Shandon, on the Gareloch. The wind during the night was 
very light and from the north, but fell calm in the morning, and the sky was cloudless. 
Over the whole Clyde area a dense fog formed, while over the Gareloch the air remained 
perfectly clear, the distant mountains being quite sharp. The only difference in the air 
over the two areas was that while the air over the Gareloch remained pure, that over the 
Clyde was much polluted by the products of the fires of the towns on the banks of the 
Clyde. The air over the Clyde might have been slightly warmer and damper than the 
other ; but as the Gareloch is tidal, this difference can have been but slight. 
