Page:Handbook of Meteorology.djvu/157

 good seeing if the air is dry, and dryness of the air, when not polluted, is fundamental to its clearness. Rain or snow, and mist are pretty certain to accompany a cyclone. A cirrus haze at the eastern horizon and a white sky overhead are followed by gathering clouds which increase in thickness, and by precipitation. These changes can be forecast, both as to time and place, with a fair degree of certainty. The advancing half of a cyclonic depression is an area of increasing impairment of seeing; the receding half is one of improving visibility. The same is true of the V-shaped depressions of western coasts in high latitudes. On the front of the V the seeing grows worse; at the rear, it constantly improves. In each case the decreasing pressure brings foul vision; the increasing pressure, good seeing.

Stagnation of the air almost always brings haziness, but rarely to an extent that interferes with good seeing. In some cases, such as the “stranded Bermuda high” it may be roughly forecast. The haziness resulting from stagnation may interfere with the long-distance helio-signals occasionally necessary, or with the long-distance sighting in geodetic surveys; otherwise, the impairment is not of consequence.