[Arran]

It isn't temperature changes that cause the celluloid dial bubbles to crack it's age and rough handling. As that stuff gets old it yellows, and becomes brittle, it isn't unusual to see one with a jagged hole punched right through it like a pain of glass. Of course ultraviolet may play a role as well, such as if the set was kept near a window, in this case the lacquer looks like it was sun damaged as well.
Regards
Arran