Originally posted by NormChris
Specific humidity is more exact. Specific humidity is like saying there are 4 ounces of water in an 8 ounce glass. The glass is still half full but now we are specifically stating exactly how much water is in the glass.

The relative humidity of air is stated in %. Specific humidity is stated in grains of water vapor per pound of air. It can also be stated in decimal fractions of a pound of water vapor per pound of air.
I prefer humidity ratio, mass of water to mass of dry air, since this scale is used on the standard ASHRAE psychrometric chart. But specific humidity, mass of water to mass of moist air works. They're related as follows:

sh = W / (1 + W)

where:
sh = specific humidity
W = humidity ratio

and

W = Mw / Mda

where:
Mw = mass of water
Mda = mass of dry air