1) Hygrometer 
instrument for displaying changes in air humidity
• (n.) An instrument which shows whether there is more or less moisture in the atmosphere, without indicating its amount.
Hy'gro·scope noun [ Greek ... wet + -scope : confer French hygroscope .] (Physics) An instrument which shows whether there is more or less moisture in the atmosphere, without indicating its amount. 
[n] - hygrometer that shows variations in the relative humidity of the atmosphere
hygroscope An hygrometer, or an instrument that indicates changes in atmospheric humidity, that shows variations in the relative humidity of the atmosphere. Unlike a hygrometer, a hygroscope only indicates a change in relative humidity, without measuring the magnitude of the change.
noun hygrometer that shows variations in the relative humidity of the atmosphere
an instrument that indicates the approximate humidity of the air.