A clock hygrometer is an instrument that can measure both time and relative humidity, which is the amount of saturation in the air. One option is a dial model, which typically has two dial faces with needles pointing to the time and percentage of relative humidity. Another type is a digital clock hygrometer, which is an electronic device that usually has a liquid crystal display (LCD) or similar screen that displays the numerical values for each measurement.
The clock portion of a clock hygrometer shows the current time. Some clocks will display the time in 24-hour format, sometimes called military time, and others use a 12-hour format. Digital clocks might also indicate if the time is A.M., before noon, or P.M., after noon.
Relative humidity, which is another thing that a clock hygrometer measures, refers to the percentage of water vapor in the air. The air can only hold a certain amount of water at each temperature, so a hygrometer compares how much moisture is in the air to how much the air can hold at that temperature. The humidity range when humans generally feel the most comfortable is between 30% and 70%.
A dial clock hygrometer has two dials — one for time, and one for humidity — with needles that point to the current measurements. The clock will have needles for the hours and minutes, and some will have a needle to also show seconds. This type of mechanical clock might use mineral quartz to keep time or a mainspring that unwinds, which causes the gears inside to turn and make the needle on the face of the clock move. A dial hygrometer uses a hair that expands or contracts with changes in moisture, triggering a lever to move the needle on the face of the dial.
A digital model of clock hygrometer typically uses batteries for power. The electronic clock components include a counter to keep time so that the current time can continuously be displayed on the LCD screen. The hygrometer works by measuring changes in electrical resistance caused by moisture in the air. In some cases, the clock hygrometer might include a wireless sensor that can be placed in a separate location from the main unit so that the relative humidity of two points, such as indoors and outdoors, can be displayed. Also, some digital models will show additional measurements such as barometric pressure and temperature.