This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
33. The water vapor which pervades the atmosphere exists in the form of a gas independently of the oxygen and nitrogen, but, being a compound gas, its properties differ somewhat from those of a simple gas. Thus, it condenses into water at 212° under a pressure of 14.7 pounds per square inch, while oxygen and nitrogen become liquids only at extremely low temperatures, and under enormous pressures. This vapor, or steam, does not, however, condense entirely and completely into water under atmospheric pressure at the temperature of 212°. A part of it remains in the gaseous condition as true steam, even though its pressure should fall below that of the atmosphere. Steam exists at all temperatures down to zero, and even many degrees below. Thus, at 20° below zero, and under an absolute pressure of .008 pound per square inch, steam still exists. Under natural conditions, the atmosphere is never free from the presence of steam or vapor of water.
The absolute pressure of this steam is very low, but it forces its way into the space which is occupied by the other atmospheric gases, and increases the total tension of the atmosphere by the amount of its own pressure. The aggregate tension of the gases that constitute the atmosphere is shown by the barometer, but no instrument has yet been devised that will show the actual tension of any single one of these gases; therefore, the pressure of the atmospheric steam cannot be measured directly, except by apparatus which cannot be conveniently used outside of the laboratory.
A cubic foot of air will admit or take up the same quantity of steam as a cubic foot of empty space. The weight of the steam will depend solely upon the temperature, provided that it does not become superheated. The temperature to be considered is that at which condensation begins.
34. The ratio between the quantity of vapor, or atmospheric steam, which is actually present in the air, and the maximum quantity which it could contain at the temperature and barometric pressure then prevailing, is called the humidity of the air.
When the atmosphere contains the maximum quantity of steam that can exist at the temperature of the air, it is said to be saturated with moisture. During fair weather the quantity actually present is much below the maximum that the temperature of the atmosphere would permit. When the maximum is reached, the least diminution of temperature or barometric pressure will be followed by the condensation of a part of the vapor. The condensed vapor will be precipitated as dew or rain during summer weather, or as snow in winter time, and in very cold weather it will appear as hoar frost.