I wouldn't know about sultry weather, as we don't get any in and around San Francisco, but as for other uncontrollable variables I would say:
1. Wind direction and strength. In the morning it is usually still, but the ocean breeze (aka fog) kicks up on summer afternoons and my evening commute went from cross/head winds for the first 10 miles, side/cross/tail winds for the next ten, and cross/tail winds for the final 10. Typically on each 10-mile leg I would get 70, 90 and 100 mpg on a summer run that started off cool in San Francisco and ended hot on the peninsula; which brings me to . . .
2. Temperature - hotter is better, cold in an mpg killer, which includes a/c, which itself can knock off 10 mpg. Roll up the windows and as long as the temperature inside didn't fry my brain I was cool with that, even if I did have to take a cold shower when I got home.
3. Not just hills and dales, but also the road surface, the smoother (quieter) the better. This can be a 5 mpg variable. There is a section on 101 by 92 that is a magic carpet ride.
4. Rain sucks, its like wading uphill through treacle, but mercifully we don't get a whole lot of it.
I have moved and changed my commute, which is now much shorter and hillier and the traffic is more viscous and my mpg has fallen off a cliff, so maybe its time to lube up the old penny farthing . . .