THE MEASUREMENT OF HUMIDITY: HYGROMETERS
The Water-vapor Content of the Air.—Water vapor exists in the troposphere, or lower shell of the air, at all times, the amount depending very largely on the temperature of the air. To the best of knowledge, little if any water vapor exists in the air of the stratosphere. The high cirrus clouds presumably mark the upper limit of the condensation of the water vapor of the air.
The maximum proportion of vapor—that is, the maximum quantity per unit of volume—depends on temperature; it is independent of the other constituents of the air. Were there no other constitutents, the atmosphere would be an atmosphere of water vapor, and the amount per unit of volume would be about the same as under existing conditions. It is best, therefore, to consider the water vapor content as an. independent factor so far as measurements are concerned.
When the air—or rather, the water vapor itself—is near the point of saturation it is moist to the senses. Hygroscopic substances, such as sugar, salt and many other substances, absorb moisture; sized paper and starched fabrics swell and become limp. These conditions begin to be noticeable when the water vapor of the air passes 85 per cent of the amount that may exist.
When the vapor content is 30 per cent, or less, of the amount required for saturation, the dryness becomes apparent to the senses, especially to the lips and throat. The gummed surface of stamps, labels and adhesives shrinks, causing the paper to curl. Doors warp and thin panels of wood shrink and split.
The moisture sensation of the air is its humidity. Before saturation is reached, the moisture is in the form of vapor; at the point of saturation it may appear in the air as fog, or cloud; when the ground temperature reaches the point of
