Page:Handbook of Meteorology.djvu/152

 If an iceberg is sighted dead ahead during foggy weather it is usually so near that avoidance is difficult. A steamship can stop or it may back. An air plane can do neither.

From time to time experiments in oiling the area of fog-covered waters have been tried; but the oil film has no effect on the fog. The fog comes from the air and not from the water and the oil film does not warm either one.

The fogs of the Pacific Coast occur usually in early evening and may continue after sunrise. In southern California the coast fog may be high above ground. It is then the “velo,” or veil. The fogs of the middle Atlantic Coast are usually associated with cyclonic movements. To a certain extent they are of the nature of city fogs, being encouraged by the smoke and dust incident to city industries. In the larger harbors fog may be forecast, but not with a high degree of verification. Even a light fog ties up shipping pretty effectually.

A light fog may be more opaque than a heavy rainfall. Direct rays of light do not penetrate a dense fog more than a few rods. The light is scattered by reflection. The amount of water contained in a cubic foot of saturated air at 67° is 6.2 grains. If the temperature be reduced to 42°, approximately one-half the vapor, 3.1 grains, will appear as fog, and this amount is sufficient to produce a very dense fog.

In practise, a single rule must guide a pilot; when the limit of visibility is less than the distance required to make a stop, there is danger. In traversing sea fogs, where other vessels are not likely to be encountered, sailing masters have expressed the opinion that quite as- much danger exists at half speed as at full speed. Perhaps this is true if one considers a collision. Nevertheless, at full speed an average of twice as many chances of collision will occur, for the vessel will meet an average of twice as many other vessels in a given time.

To the airman there is no difference between fog and cloud, so far as the impairment of visibility is concerned. For the greater part, a pilot may fly above stratus clouds if the air is not clear below them; and airmen usually can find plenty of room under the alto-stratus clouds of an overcast sky. But an airman who has once encountered a cumulo-nimbus cloud, or even large masses of low cumulus clouds, is not apt to repeat the experience. In many cases both fog and cloud may be