Question

what is the measure of moisture in air

Answer 1

A device used to measure humidity is called a psychrometer or hygrometer. A humidistat is a humidity-triggered switch, often used to control a dehumidifier. Humidity is also measured on a global scale using remotely placed satellites.

Answer 2

Moisture is present in very small amount i.e about 0.04 - 0.06 %
So, its weight is also less and its volume is less ultimately