Unclean burn rate is most likely the culprit. Using the same fuel as always? Might be wet or moist, which is what causes white smoke in my stuff. Also using too much or wet smoking wood will do the same. Good luck. Steve.
^ +1 could be wet or an unclean burn as mentioned above.
But if you have drippings that are hitting the coals this could also cause the problem. The drippings won't make the food bitter, it will just make smoke and maybe next time fashion a drip pan if there is room.
I'm with Blu, seems like it has to be from high humidity and cold temps. I would think that at some point in the 6 hours the burn would settle in and produce some thin blue if it was a burn problem. I always get white for the first 45 minutes while my smoker is heating up. And I'm always hoping like hell no one is up yet judging me. :laugh:
HAHA!! Kinda reminds me of my neighbor that always goes out and starts up his grill for burgers like 15 minutes before its time to eat. He owns stock in lighter fluid... Not sayin that you do that, just made me think of that guy...