What is a psycrometer?

Answer 1

This is an instrument used to measure the relative humidity in the air.

[Another way of determining this is to measure the dew point of the air. The temperature at which dew will form on a clean mirror that can be chilled. ]

Answer 2

A psychrometer is a type of hygrometer used to measure the relative humidity of the air. It consists of two thermometers, a wet bulb and a dry bulb, mounted side by side. By comparing the readings of the two thermometers, the relative humidity can be calculated.