In the ini files there is this line:
;; generate new cloud cover every N hours
WeatherRandomizeInterval = 7
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
If you change the number then it will change the hours that the weather is randomized. I also read it was best to keep it an odd number. This was from Stw's post and site, I believe.
Also, at modthesims there is this:
http://www.modthesims.info/showthread.php?t=426867
I haven't messed with them and you reminded me I wanted to change the hours of sunrise and sunset in my last world. :lol: