I know what you mean. Sometimes you get an odd mix of weather/humidity which causes weird behaviour and the blowers / demister struggles to have any effect. I had an odd occurrence whilst driving yesterday. Had been driving about 20 mins, windows all clear, air con off, then it started thundering and lightning and car instantly steamed up on all windows. It got so bad I nearly had to pull over as visibility was getting really bad, on a dual carriage way at the time so not ideal!! In the end the only thing that helped was opening the windows (and getting soaked in the process) which cleared it.