The relative humidity of the atmosphere is the amount of moisture contained in a given space as compared with the amount the same air could possibly hold at that temperature. Warm air will hold more moisture than the same air when cold. Air absorbs water like a sponge to a point of saturation. When the air is filled with moisture, any change which takes place to reduce the temperature also reduces its capacity to hold the water vapor and the excess is deposited as dew. This supersaturation ordinarily takes place near things which lose their heat faster than the surrounding air and the nearest colder surface acts as a condenser to receive the drops of dew. Grass being in convenient position is the commonest receptacle for dew formation. If the dew forms in the air it falls as rain, but if the temperature of the dew-point is below freezing, the dew immediately freezes and snow is the result.
In the consideration of problems that involve atmospheric moisture, both relative and absolute humidity are factors of common use, that are capable of exact determination. The relative humidity of the air is most readily determined and as it expresses the state of the atmosphere in which plants and animals live and thrive, as opposed to other conditions of humidity in which they sometimes sicken and die, it is one of the indicators of the quality of atmospheric air.
In the subject of ventilation, which is undertaken later, it will be found that a definite knowledge of atmospheric humidity has much to do with the successful operation of ventilation apparatus. Most people recognize the “balmy air of June” without realizing just why at the same temperature other seasons are not so delightful. In reality it is the condition of atmospheric humidity combined with an agreeable temperature that gives the kind of air in which we find the greatest degree of comfort.
The effect of moderately warm humid air is that of higher temperature than the thermometer indicates. When the atmosphere is near the point of saturation, the evaporation which usually goes on, from the surface of the body, practically ceases. In summer time a temperature of 85°F. with relative humidity of 90 per cent. saturation seems warmer than a temperature of 100° at 40 per cent. saturation, because of the cooling effect produced by the increased evaporation due to the drier air.
In winter, when most of the time is spent indoors, in an atmosphere that is very dry, the sensation of discomfort produced by the lack of humidity oftentimes leads to physical derangements that would never appear under more desirable conditions. The cause of many ailments of the nose, throat and lungs during the winter months is attributed by physiologists to breathing almost constantly the dry vitiated indoor air. The cause of dry air in buildings is not difficult to explain; it is a great deal more difficult to realize that the lack of water breeds so much discomfort.
In order to express the condition of humidity that may exist in the average dwelling, office or school-room during the winter, it is most convenient to refer to the results of varying atmospheric conditions that are given in Table 1—Properties of Air—which appears below. In the second column of the table, under the heading “Weight of vapor per cubic foot of saturated air,” will be found the amount of moisture in grains per cubic foot that will be required to humidify air at different temperatures. It will be seen that at 10° the air will contain, when fully saturated, only 1.11 grains of water, while at 70° temperature the same air would hold 8 grains of water. These amounts will be found in the column opposite the temperature readings. It is at once evident that when saturated air at 10° is raised to normal temperature 70°, the original amount of moisture is contained in an atmosphere capable of holding 8 grains of water. Its relative humidity will therefore be 1.11⁄8, practically 14 per cent. saturated. Unless moisture is received by the air from some other source this condition will produce a very dry atmosphere.
The normal atmospheric temperature of 70°F. with a relative humidity of 50 to 60 per cent. saturation produces a condition that is one of agreeable warmth to the average person in health and is recognized as the atmosphere most desirable. To some, this state of temperature and humidity is that of too much warmth and a temperature of 68°, with the same humidity, is most agreeable. At the same temperature, a reduction of the humidity to 20 per cent. saturation will produce a feeling of discomfort and the sensation will be that of a lack of heat. The cause for this latter feeling is due to excessive evaporation of moisture from the body.