When the weather is either very humid or quite cold, I find working under the dark cloth to be very difficult. Since I can't break the habit of breathing (I've done it all my life), the groundglass, my focussing loupe, my eyeglasses, and everything else in sight fogs up. Every few seconds, I've got to stop and wipe off the fog. It even affects my equipment choices-- I use a relatively inexpensive loupe because I don't want to ruin a good one by wiping it with my shirt, handkerchief, or whatever is handy. How do other people deal with the fog problem?