avoided; but one cannot avoid a fog when it shrouds a landing place. And while fog turbulence is slight, cloud turbulence may be very great, and this is notably the case with cumulus, and cumulo-nimbus clouds.
It is not easy to draw the line between fog and mist. Inasmuch as the droplets of mist are much larger than those of fog, they do not scatter so much light; moreover, measured per cubic unit of air, there are not so many of them. At times one may see a gray moisture haze in the direction of the horizon. A little experience enables one to distinguish it from a dust haze. It is never thick enough to impair seeing materially.
Humid air is not quite so clear as dry air, but it rarely loses transparency to the extent that it impedes transportation. Nevertheless, the impairment of visibility by air that was merely very moist has been the critical point of several suits in which railways were involved.
Rain and Snow.—It is not often that rain, per se, falls so fast that the seeing is badly impaired; but now and then this happens. Very heavy downpours may limit the vision to less than a few rods. But downpours of this sort are not common, and if the seeing is passable for 1000 feet ahead, danger is largely avoidable.
Some of the light passing through raindrops is refracted; some is reflected and otherwise scattered. The outlines of an object which normally is distinct may be obliterated, but its mass is likely to be seen. In this respect rain differs from fog. Even if the combined surface of the drops next the observer is sufficient to form a screen, the screen is partly transparent, but a fog screen is practically opaque. If a rain-drop be broken into water particles of fog size, their aggregate surface is several million times that of the rain-drop. The screening power of the fog, therefore, is vastly greater than that of the rain-drop and so also is the amount of light scattered.
A fast-falling snow is about as bad for visibility as an ordinary fog. If blizzard conditions prevail, the snow may be broken into a fine ice dust quite as opaque as a thick fog. The airman may avoid a snow squall of small area by flying around it; the locomotive engineer and the marine pilot must push through it. The danger point is reached when a snowfall hides semaphores or obscures signal lights.
