Personally, without being a biologist, I think the photoperiod is the primary stimulus, and is what I've read in the past by those who should know. However, I do think weather can be a factor, which makes sense, if you think about it. A prolonged period of cool, overcast days, could, in my opinion, slow things down. I've never read anything about the intensity of the daylight, but to me it makes sense that bright, sunny weather is going to trigger a greater response than dreary days. I know it does in humans. Virtually all of us feel better when the sun is shining.