What's the highest ambient temp you've ever seen ice form on the wing when parked at the gate? Two years ago I was on a DL plane at SLC and we had to de ice despite the fact that it was about 48F outside. I wouldn't have believed it myself but I actually saw the sheet of ice on the wing, after the guy next to me pointed it out to the FA. The pilot didn't believe the guy and came back to look out the window on his insistence, and sure enough we de-iced.