Page:Handbook of Meteorology.djvu/146

 visibility, and these are all contained within the air itself. The factors which impair visibility are:


 * Fog.
 * Thickly falling snow.
 * Dust storms in arid regions.
 * Very fine rain—many drops per cubic foot.
 * Heavy rain—large drops. Dust storms in swarded regions.
 * A smoke pall held down by a “lid.”
 * Moisture haze in a smoky air.
 * Moisture haze or dust in clear air.
 * Refraction caused by the mixing of warm air and cold air.

Experience and judgment have taught pilots how to avoid dangers that confront them. It remains for meteorologists to fix definite standards and to express varying visibility in terms that are comprehensible and intelligible to all. Moreover, in many instances, changes in visibility may be forecast with a high percentage of verification.

Fog is generally regarded as the chief factor in the impairment of visibility, but the various other factors cannot be arranged in an inflexible order. A desert simoon may impair visibility quite as much as the worst fog; but simoons are rarely in the van of transportation, while fog is practically coincident with every line over which commerce is carried. On the other hand, though the blurring of outlines by refraction, and the slight discoloration of the air by a dust haze may reduce transparency, neither one is a menace to safety. Not even a moisture haze is disconcerting unless it conceals the horizon.

In general, the impairment of visibility is due to various movements of the air within itself. Except as the wind picks up fine dust, nothing is added to or taken from the air to make the difference between transparency and opacity. The great planetary movements of the air need not be considered here. The cloud belts incident to them and the conditions which produce precipitation are fairly understood; they are regular and periodical. Such movements as are not planetary—the cyclones and the anticyclones are understood as to cause and effect, and the forecasting of them has become a science. But there are other movements, more or less superficial and local, that are not so well understood. The causes of them may be known,