THE THERMOMETER FOR HUMIDITY IN THE AIR
Moisture or dampness in the air, as shown by an instrument called the hygrometer, increases before rain, fog, or dew.
Before describing the hygrometer, a definition of a few of the terms used in conjunction with the instrument will be found useful.
ABSOLUTE HUMIDITY
The amount of vapor actually present in the atmosphere is termed the absolute humidity, expressed usually either in the expansive force that the vapor exerts or in its weight in grains per cubic foot of air.
RELATIVE HUMIDITY
The absolute humidity divided by the amount of vapor that might exist if the air were saturated gives a ratio that is called the relative humidity.
DEW POINT
The temperature at which moisture begins to be condensed on a cold vessel or other container and becomes visible is called the dew point.