Vili 
Supplement to “ Nature,” April 14, 1923 

this contaminated air mixed with the pure air condensa- 
tion took place and the long strip of mist resulted. 
This observation furnishes the explanation of an old- 
standing difficulty in meteorological optics. Cirrus 
and other very high clouds frequently exhibit a most 
beautiful colour effect, which has received the name 
iridescent clouds, as the clouds appear to be iridescent 
with colours like those of mother-of-pearl. If cirrus 
clouds are always composed of ice particles, as one has 
generally concluded on account of the low temperature 
of the atmosphere where they form, the colours cannot 
be satisfactorily explained ; but if they are composed 
of water drops the explanation is easy. If water drops 
can exist at — 30° C. near the earth’s surface, there is no 
longer any reason to postulate ice crystals for the cirrus 
clouds ; and we may now say definitely that iridescent 
clouds prove that cirrus clouds are sometimes composed 
of spherical water drops. But it also proves that air, 
especially in the upper atmosphere, is frequently devoid 
of solid nuclei on which condensation can take place. 
Haze.—Perfectly pure air is almost completely trans- 
parent to visual light waves, and if the air were always 
pure we should see distant objects through air almost 
as clearly as through a vacuum. But the air is never 
pure ; there are always more or less particles of foreign 
matter present. The action of these particles is two- 
fold: first, they reduce the amount of light reaching 
the eye from distant objects ; and, secondly, in the day- 
time they scatter the general light of the sky and so 
send to the eye extraneous light which reduces the 
contrast between distant light and dark objects on 
which visibility depends. Generally this foreign matter 
consists of a mixture of solid ponderable particles and 
hygroscopic molecules. The latter in perfectly dry 
air would be practically invisible, but when loaded with 
water in a humid atmosphere they add to the obscurity 
of the atmosphere. 
The amount of obscurity will therefore vary with the 
amount of solid matter and with the humidity of the 
air. Haze is due to this kind of obscurity, and varies 
in intensity from the slight obscurity of polar regions, 
which depends almost entirely on the hygroscopic 
particles, to the dense obscurity of a dust storm in 
tropical regions, which is due almost entirely to solid 
particles. 
Mist.—When the temperature of air falls, the 
humidity increases until the saturation point is reached. 
The diameters of the hygroscopic particles grow, but 
even in saturated air the amount of water extracted is 
not great, and if there is little solid matter present the 
obscurity is not marked. But if the temperature falls 
below the dew point the hygroscopic particles are 
sufficiently large to form excellent nuclei for condensa- 

tion, and relatively large amounts of water are de- 
posited for small falls of temperature. 
Real condensation has now commenced, and the 
obscurity changes from that of haze to that of mist. 
It has been a common practice*to record atmospheric 
obscurity as haze when there is a noticeable difference 
between the readings of the wet- and dry-bulb thermo- 
meters, and as mist when the readings are the same. 
This, however, is not a true criterion, for the air can be 
saturated without condensation, while mist cannot be 
formed until water has been condensed on account of 
a fall of temperature after the dew point has been 
reached. The whole process of the formation of haze 
and mist is continuous, and in practice it is practically — 
impossible to say when haze becomes mist, although 
extreme cases are easily distinguished. Nevertheless 
haze and mist are fundamentally different, for haze 
owes its origin to foreign matter, and the small amount 
of water associated with hygroscopic nuclei, while 
mist is due to an actual precipitation of water from 
vapour to liquid. 
Fog.—There is, however, no fundamental difference 
between mist and fog: in most cases fog is only a 
dense mist, and the density at which mist becomes fog 
is a matter of definition. It is now the practice of the 
London Meteorological Office to limit fog to the obscurity 
in which objects at one kilometre are not visible. 
When mist and fog are formed in fairly clear air they 
are white. On the other hand, if the air contains a 
large quantity of impurities, such as carbon particles 
from imperfect combustion, the mist particles absorb 
the impurities and become themselves dark-coloured. 
In this way are formed those dense fogs in London 
which are likened to pea soup. It was originally 
thought that the density of a London fog was due to 
the fact that the smoke of the city provided an un- 
usually large number of nuclei on which condensation 
could take place, thus offering a temptation to the air 
to deposit its moisture which it could not resist. As a 
matter of fact, there are always sufficient nuclei in the 
purest air in England to allow of the formation of fog 
whenever the meteorological conditions are suitable. 
The relationship between smoke and fog is peculiar, 
and may be said to be accidental. The meteorological 
conditions which are necessary for the formation of 
fog are such that while they last smoke cannot get 
away either vertically or horizontally from the place of 
its origin. Above the fog there is a temperature in- 
version which effectively prevents all upward motion 
of either air or smoke, while fogs over the land usually 
form in calm air. Thus during a fog practically all the 
smoke which London makes is kept over it and within 
a few hundred feet of the ground. This smoke, com- 
bined with the deposited water, can, as we all know, 

