Sun is better than rain but rain is better than wind. Wind can shut you down pretty quickly. This is one of the times when I think the electronic caller does have a legit advantage. You can crank up the volume and try and get that sound out there if the wind isn't too bad.
Right after the rain or a good snow is a great time to call though. Lots of hungry critters that have been holed up for a while.
Songdog's reasoning is validated by my cam pics of coyote's. More often than not when i get coyote pics they are right after a decent rain - not always though. I'd say 70% of my coyote pics were after a rain, 30% other times.