Dust Storms.—In arid regions winds of the simoon type are not uncommon. Frequently they carry heavy clouds of dust far beyond desert boundaries into fertile regions. The Santa Ana of southern California is an example. Dust storms originating in the plains states sometimes carry their content as far east as the Mississippi. So far as the impairment of seeing is concerned their effect does not reach more than a few hundred feet above ground. The haze of fine and highly electrified dust which commonly hangs in the air after a dust storm is not very opaque, but it extends much higher above ground. It may persist for several days. The airman may fly above a dust storm, but if it blankets a landing place it becomes a positive danger.
The ordinary dust haze, a bluish tinge observable against a dark background, does not impair seeing. Frequently it has the density which the landscape artist terms “atmospheric effect.” The sea haze, on the other hand, may be disconcerting because it may hide the distinctive marks of nearby vessels. The sea haze has been an interesting factor in naval strategy because of this fact. Frequently it is dense enough to interfere with signaling, even with helio-apparatus. Calmness of the air is a condition necessary to the formation qf the blue dust haze and the sea haze.
The Smoke Pall.—The smoke that hovers over manufacturing districts differs materially from that of forest fires, being composed largely of free carbon and hydrocarbons. In moist weather water drops varnished with tarry matter are mixed with smoke carbon. Mixed with stack products are sulphur gases—sulphur trioxide and sulphur dioxide. These are chemically active and in the presence of moisture become very effective nuclei of condensation.
In manufacturing districts where soft coal is extensively used as power fuel the smoke pall may be dense enough to hide the outlines of large objects at a distance of 4 or 5 miles. A low-lying lid holds the smoke pollution close to the ground and impairs seeing very materially. The airman may fly above the lid into a region of clear air; the marine pilot, except in harbors, is out of the way of smoke; the locomotive engineer, who cannot avoid the smoke pall, sometimes finds it disconcerting. It rarely interferes with signaling.
