Page:Handbook of Meteorology.djvu/20

8 in winter than in summer. The proportion is greater on clear, dry days than during cloudy spells. The daily variations of the ozone content of the atmosphere seem to correspond to the variations of the atmospheric electric potential.

Dust particles so fine that they escape measurement even with the highest power of the microscope, must be considered a part of the normal content of the atmosphere. Their presence is indicated by the fact that they may reflect enough light to make them visible en masse when a powerful light is turned upon them in a darkened room, or when a searchlight throws its beam at night. The path of the light is shown by the light reflected from dust motes. Dust particles of the size thus revealed behave like molecular rather than like matter of molar sizes. They are floating matter, the particles of which may not settle unless they are brought to the surface by means other than their own gravity.

The floating dust motes of the air are factors of great meteorological importance. They are the nuclei upon which the water vapor of the air condenses. Dense clouds of volcanic dust act as a screen preventing much of the sun’s heat from reaching the earth. The dust particle is the normal nucleus for the cloud particle. The flying, or windblown dust, though a highly important physiographic agent, is not a factor of importance in meteorology.

Chlorine usually occurs in the air of localities bordering upon the oceans, and sodium chloride reactions may be obtained when sea winds are blowing inland. The presence of the salt is due to the action of the wind which whips a small amount of spray into the air. The chlorine content of the air apparently plays no part in meteorology. Like smoke and chimney products it may be regarded as “foreign” matter.

From the foregoing it is apparent that oxygen, nitrogen, and the argon group of gases practically constitute the “fixed” constituents of the atmosphere. Their proportions at sea level vary but little in different parts of the earth, and they constitute about 98 per cent of the atmosphere. Ozone, the nitrogen oxides, ammonia and the various radio-active emanations may be considered practically as negligible factors in meteorology; for the greater part they are accidental. Carbon dioxide is a factor chiefly in physiological meteorology.