Plate 47.
FALL CLOUD.
(Stratus Lenticularis.)
A curious feature is sometimes shown on the underside of a thick cloud, which is probably due to the upper part of the ascending column having been carried beyond its position of equilibrium by its own inertia, and then falling back again in the teeth of the still rising lower part. The result is to give the base of the cloud an appearance of a number of rounded masses hanging downwards below the cloud, very suggestive of the idea that the cloud is upside down. Such an event will not often occur, and when it does the conditions are quite wanting in stability, and the consequent features will be very transient. When the base of a cumulus or cumulo-nimbus is so affected, the cloud is known as festooned cumulus, or cumulus mammatus. A precisely similar structure may be seen under strato-cumulus, or even thick stratus. In some countries it seems to be frequently observed, but in England it is so uncommon that the writer has only noted it about a dozen times in twenty years, and on no one of these did it last long enough to allow of its portrait being taken. It is an indication of very disturbed conditions, and is usually followed by heavy rain.
When cumulus clouds are formed in air which is steadily moving as a whole, that is to say, when there is a steady breeze, they have a very decided tendency to follow each other in long lines. It may often be noticed that in a particular place with a certain direction of wind these long processions follow definite tracks in relation to the geographical features. The phenomenon does not seem to have been recorded except in hilly country, but has frequently been observed by the writer. It is not the same thing as the formation of stationary belts of cloud transverse to the wind. These cumulus float along with the movement of the air, and the question to be answered is, why should they follow each other so persistently, and why should the intervening belts of sky be so continuously free from cloud.
If we consider that the warm damp air which supplies them is drawn from the ground, it seems that any cause which tends to direct this warm stratum into definite channels, as it is carried on by the wind, will be a competent cause of the whole phenomenon. This we find in the presence of lofty hills which stand in the way of the warm surface winds, causing them to follow more or less the general trend of the valleys, and so delivering the rising convection currents of cloud-producing air at the same spot.
It is easy to conceive that other causes, such as a difference in temperature or dampness of neighbouring tracts, resulting from whether they are bare or wooded, marshland or sandy plain, might equally suffice; or might, at least, powerfully co-operate with, or counteract, the effect of hill and vale. But in any case it is plain that the geographical conditions to the windward of the place of observation not only may affect the occurrence and distribution of cloud, but if the wind is steady it is difficult to see how they could avoid affecting it.
Another puzzling phenomenon, sometimes presented by cloud and fog, is that our instruments for detecting humidity show that the air within them is not always fully saturated. It seems probable that this is due to such cloud or fog having begun the process of drying up, or that in some way not fully understood the presence of the cloud particles after they have first come into existence may cause the withdrawal of some of the moisture from the intervening damp air. The surface of each minute droplet exerts a pressure on its interior similar to the pressure exerted by the film of a soap-bubble on the air within it, and it is conceivable that some of the uncondensed vapour from outside may diffuse through this enclosing surface film, and be retained there in consequence of the pressure. If this is so, and subsequent investigation can alone decide the matter, it will follow that when once cloud production has begun it will be continued until the air between the cloud particles is reduced so far below its saturation point that the tendency of the drops to evaporate, that is to say, for the imprisoned water to escape through the confining film, balances the retaining pressure.