While it might be sunny and hot down in the valleys, the mountains can be windy, very cold, and you might find that it's even snowing at higher altitudes! When hiking (or driving the auto road to the summit of Mount Washington), it is important to keep this in mind and be prepared for any weather; you'll be much happier if you pack properly!
It's a cliché, but it's true! In the vicinity of the White Mountains, moose and other large animals seem to be as plentiful as people. When driving, particularly at night, it is important to be both alert and cautious; moose are especially active around dawn and dusk and can be extremely unpredictable in their behavior near roads. When hiking, never approach wild animals and be especially careful to never get between a mother and her young (if you run into a moose on the trail, stand in front of a tree and stay still; moose don't have great eyesight and will have trouble seeing you).