I came off the winch launch one early morning and set up to return when I noticed the variometer was showing a climb. Must be my day I thought and I attempted to "thermal". It soon became obvious that I as not going up and I made a hasty return and just got back. The ball was stuck at the top of the glass tube due early morning moisture. I should have known that a T31 would not go up on a cold damp day! This was at RAF Waterbeach in the 60s All good fun tho.