# What is the meaning of dew point -20 or -40 or -70 deg.C? Since below zero centigrade, water cannot dew (already freeze)?

I saw these specifications listed for a compressed air dryer (desiccant type). I also wonder, can we extract moisture without make it liquid (dew) first when in it is in a vapor state? How would we do this?

• I give time for other answer (if any). Your attitude is rude and downvote is inappropriate. But I will not downvote your answer, it is good. I cannot comment since I think I don't have enough knowledge, I will let others to do that. Apr 12 '19 at 10:01

Start with the phase diagram of water found at this link. The solid <-> vapor transformation occurs at pressures below 1 bar. Using a temperature of -40 $$^o$$C puts the pressure around 7 Pa. This is vacuum conditions.