Haven't heard of any studies about weather and DMD, we live in southern California desert where it can get VERY (113 in shade) hot but not normally with humidity, ours is a "dry heat". Our son is 11, my husband and I have discussed how difficult it must be to live in areas with more severe weather concerns. I can't imagine living in the snow, wind or rain and trying to function with a scooter and such. In summer we wear hats and sunglasses, swim (good for dmd) everyday from spring on, have a canopy cover for the scooter, use umbrellas for shade. We don't use sunscreen much on our son so he can get a dose of vitamin D. On extremely hot days we stay indoors with air conditioner on and play wii or watch movies. Almost all houses in our area have air conditioning, cars too. It comes as a standard item.
I think dmd is easier to manage while living here. Ice, snow and rain would make things really tough.