1911 - 12 .] 
The Sun as a Fog Producer. 
183 
XV.I. — The Sun as a Fog Producer. By Dr John Aitken, F.R.S. 
(MS. received March 2, 1912. Read March 18, 1912.) 
I fear that most, on reading the above title, will think it is either a 
printer’s or an author’s error, and that the title ought to have been “ The 
Sun as a Fog Disperser.” We are familiar with the sun’s power of clearing 
away fogs ; we so often see fogs clear away as the sun rises, that we seem 
to have shut our eyes to another side of the question, which is, that under 
certain conditions the very opposite of this happens, the air thickening and 
fogging as the sun rises. Some years ago it was noticed at Falkirk, 
especially during the winter months, that on many mornings on which the 
air was clear before sunrise it gradually thickened to a dense haze, or to 
a dense fog, while in pure country air the observer never noticed similar 
changes. The question naturally suggested itself, what was the cause of 
this difference between the air at Falkirk and that of the country ? A series 
of meteorological observations was therefore begun to ascertain whether this 
occasional fogging after sunrise was merely an accident, or if it had any 
connection with the sun ; that is, to ascertain whether its appearance was 
what we call fortuitous, or if it appeared according to any law in nature. 
As we are to be dealing with haze and fog in this communication, it 
may be as well to state, as near as we can, what we mean by the 
words “ haze ” and “ fog.” No air can be called absolutely transparent 
in the accurate sense of the word. All distant scenery is always more or 
less veiled by what we term haze. This haze is composed of an enormous 
number of very fine particles of dust floating in the air. If the air be 
dry and the number of particles small, their hazing effect is slight ; but as 
their number increases, so does their hazing effect. In addition to this, 
however, the relative humidity of the atmosphere has a great influence on 
the density of the haze. When the humidity increases the particles con- 
dense vapour on their surfaces and so increase in size and in their hazing 
effect, till when the air is saturated they grow large and form fog particles, 
and their power of shutting out the landscape is enormously increased. 
The dust particles may thus produce a haze of greater or less thickness 
according to their number, or they may produce a fog, but there is no 
hard-and-fast line between what we call a haze and a fog ; we usually 
call it fog when very thick and damp, but even here the boundary line is 
unsatisfactory, as we have dry fogs. So, again, there is no hard-and-fast 
