I think burning a leaf is just an accumulation of exposure equaling some percentage more than the exposure before the move. Latitude and altitude are factors in the intensity of the sun's rays, and the angle of the exposure, but this is my first time of a mention of humidity getting involved in that particular discussion. Maybe people discuss it regularly and I just didn't click those, IDK.
Altitude is not something that I have much experience with. If the mechanics of burning involve loss of moisture on plants and/or skin, then humidity probably is involved. The math of trying to calculate those variables would be rocket science level.
I'm not sure how the danger of going up in flames relates to superficial burning, but that's also interesting to think about.
Usually covered skin can burn in a few mins here if suddenly exposed, and more quickly as one goes south. I've gotten burned while swimming so many times, it doesn't seem like the 100% humidity of being wet can help prevent burning. It's interesting to think about. On a silly note for skin, I've been about 2 secs away from combusting on many occasions. I did seek the relief of a cold shower. Close calls. : )