From: Steve (Primus) (Avatar) 30/10/2001 5:53:54 Subject: re: STEVE'S WEATHER FAQ post id: 476429 Can someone please settle this argument I've been having with a friend... When it rains, is that "classed" as 100% humidity? 100% relative Humidity means that the air is saturated. The water vapour in the air is condensing into water droplets as fast as water droplets are evaporating. Temperature is a factor in relative Humidity. If the temperature increases so does the rate of evaporation. If the temperature falls, the evaporation rate falls. When it rains, the humidity increases because the rain is evaporating into the air. The water on the ground will also evaporate after the rain stops and the humidity will continue to rise. You will often see scud - low broken cloud - forming in rain below the main cloud. This is caused by the increased humidity of the air. After good rain, if the skies clear overnight, a fog is very likely the next morning. The only time you can be certain that the RH (Relative Humidity) is 100% is when you are in a fog or in a cloud because if the RH was lower than 100% the fog or cloud would evaporate. It is possible to super-saturate air under certain conditions if it is very clean and (probably) still. The water cannot condense unless it has something to condense onto, called nucleation sites. Usually it's dust, if not ice crystals. There comes a point when it condenses anyway. Steve is the man to supply more details. Supersaturation can occur but only in the upper troposphere. At the surface there are too many condensation nuclei.