Page:Handbook of Meteorology.djvu/139

 Dust and Condensation.—The experimental work of Dr. Aitkin showed conclusively that the moisture of the air condensed with difficulty in dustless air even when the temperature was several degrees below saturation; in normal, or dust-laden air, condensation took place readily. The repetition of Dr. Aitkin’s experiments under widely diverse conditions has left no doubt that the dust particles of the air, including sulphur gases set free by the combustion of fuel, are the most important factors in condensation. The research of C. T. R. Wilson brought to light additional knowledge; Wilson found that the passage of a beam of ultra-violet light through air caused condensation, even when its temperature was slightly below that of saturation. Saturation temperature, however, is not wholly necessary to condensation; a certain but small amount of condensation goes on below the temperature of saturation. Condensation goes on more freely when the humidity—both absolute and relative—is high.

Dust particles differ greatly as nuclei of condensation; they may be “good,” “indifferent,” or “poor.” The reason for the difference is not known with certainty. It may be that quickly cooling particles are better condensers than slowly cooling particles; it may be that a high degree of ionization favors rapid condensation; or that the more hygroscopic a particle the more freely it condenses. Each is a reasonable hypothesis that remains to be substantiated.

Barus and Pierce have shown that the dust particles over Providence, a manufacturing center, are far more favorable to condensation than those observed contemporaneously at Block Island. The reason therefor may be a difference in the chemical character of the dust particles; it may be due to a difference in the degree of ionization; it may be due to other and unknown causes.

One thing, however, is certain: The dust particles belched from the stacks of manufacturing districts are such excellent nuclei of condensation that the prevalence of fogs over such districts has given rise to the term “city fogs,” as distinguished