Hmmmm...I don't know. Now I have lots of ideas going through my head.

In the "mountain west" states -- Arizona, Colorado, Utah, New Mexico -- it's not uncommon for there to be a day with a high temp of, say, 65 degrees; and then the next day a high of 85. I have to think that a person would tan/burn faster on the hotter day (even if both days were sunny and cloudless). Maybe I'm wrong. But I seem to remember personal experiences of this being outdoors on two consecutive days when the temperatures were pretty drastically different.