Any of several instruments that measure humidity. The most common type of hygrometer consists of two, side-by-side mercury or electronic thermometers, one of which has a dry bulb, and one of which has a bulb wrapped with a wet cotton or linen wick. As water evaporates from the wet bulb, it absorbs heat from the thermometer, driving down its temperature reading. The difference in temperature between the two thermometers is then used to calculate the relative humidity. This type of hygrometer is also called a psychrometer. Other hygrometers make use of the temperatures at which dew forms and disappears to calculate the relative humidity. Older hygrometers used the length of a strand of hair, which stretches when it absorbs moisture, to measure relative humidity.