I have a slightly different take on cracks developing in solid wood guitars. I agree it's humidity/heat related but I feel it's rapid changes in those two factors that usually lead to cracking. I bought a '70 D28 in '86 from a seller who kept it in his air conditioned house all the time. The night I bought it was a very humid evening, and after buying it I took it to a jam an an open air house. It spent a few hours in a pretty humid, warm environment. Later I brought it home to my air conditioned house and sat it in my living room in the case. After about a half hour I heard a ping through the case. I found the dreaded B string crack from the pickguard being glued to the bare top (Matrin's after '84 don't have this issue). I got that repaired and it is the only crack it has suffered. I have 17 Martins from 1923 to 2016 and that's the only one with a crack I created myself. I humidify my home in winter but only in the 35-40% range. I've had no issues with cracking and they sound crisp and bright in that environment. My feeling is if you humidify to 50% in your home, and then take it to a place that is very dry and/or colder you will have a better chance of your guitar cracking than I will. Again I feel it's rapid changes that usually do the damage.