Absolute humidity is the amount of water in a certain volume of air
e.g 2 kg/m3 would mean 2 kg of water per cubic metre of air
Relative humidity is the fraction of humidity as a total of the maximum humidity (ie. the saturation point).
So say if at a certain temperature air would saturate at an absolute humidity of 10kg/m3, our example above with 2kg/m3 would be 20% relative humidity.
Relative humidity can be worked out by finding the air temperature and dewpoint (and converting via a chart), usually with a wet-bulb/dry-bulb device as in a Stevenson screen.
The figures in the above are more for demonstration than realism.
cheers!
foggy.
[ 17 November 2001: Message edited by: foghorn ]