Its the amount of sunlight you receive, not so much the temperatures although long periods of darkness are more closely associated with cold climates. In Alaska during the winter months when you may not get 4 hours of sunlight a day for months on end they recommend that you spend time in tanning booths. I have lived almost as long in cold climates as I have in warm (Florida, New Orleans vs Baltimore, Asheville NC and now East Tennessee) and I gotta tell you, anything belong 70 is to be avoided!!