Although the above answers are probably dead on, I would add that air humidity will effect the temps in a cooker and moderate them to an extent. Water pans in cookers achieve that, as well as aiding in the meat's moisture retention. However, if you noticed more thick, white smoke than usual during your last cook, then the culprit is definitely damp fuel. Keep your charcoal under your bed or in a coat closet maybe but definitely get it out of the basement. The charcoal should be fine if you can get some dry air to it and dry it out. Perhaps you can pour it out onto an old towel or something and let it air out on the next sunny day.
Never Trust A Skinny Cook!!!