If you leave the cap off off you fuel and leave it in the hot sun, it won't be nitro cooking off that will be the problem, and no it won't be moisture from the air either. Nitro is a dense fuel that has a flash point of about 96 degrees, I don't remember what methanol is but belive its below 0. This means you can pour nitro in a cup and put a match to it the match will go out, if it is below 96 degrees. In the hot sun the alcohol will be evaporating rapidly, rapid enough that air will not be able to enter the jug. If you did this for a time, I don't know how long, your fuel will have too much oil and nitro in the mix and not enough alcohol.