drosometer

from The Century Dictionary.

• noun An instrument for ascertaining the quantity of dew that condenses on a body which has been exposed to the open air during the night.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Meteorol.) An instrument for measuring the quantity of dew on the surface of a body in the open air. It consists of a balance, having a plate at one end to receive the dew, and at the other a weight protected from the deposit of dew.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A device for measuring the amount of dew on a surface.