You might find these two threads interesting:
http://ecomodder.com/forum/showthrea...her-17819.html
http://ecomodder.com/forum/showthrea...cts-20561.html
I've been keeping weather data in my daily log for about 18 months now. Temperature has been the biggest factor that I've noticed (about 1% difference for every 2 deg F temperature change). So far I haven't been able to notice a strong dependence on humidity. That being said, I haven't gone through all the statistics in a while. Hmm, might be time for me to do some more number crunching...