instrument for recording variations in atmospheric humidity
• (n.) An instrument for recording automatically the variations of the humidity of the atmosphere.
Hy'gro·graph noun [ Greek ... wet + -graph .] (Physics) An instrument for recording automatically the variations of the humidity of the atmosphere. 
hygrograph An instrument for automatically registering the variations in air humidity.
A recording instrument for measuring the humidity of the air.
a self-recording hygrometer.