noun, plural: hygrometers
Any of the various meteorologic instruments used for measuring the relative humidity of the atmosphere
The simplest form of hygrometer is a psychrometer, especially the wet- and dry-bulb psychrometer.
Hygrometers are used to determine the moisture content of the atmosphere. Thus, they are essentially used by meteorologists in predicting rainfall and other weather conditions.
Word origin: Greek hugros (wet, moist) + Greek metron (measure)