Better when it rains?

Ivan_nosnibor is on the right track in my opinion. However this goes beyond your own home, to include the local AC grid which I suspect benefits from the additional moisture and improved grounding when it rains.

During the summer months the rain also tends to cools things off, suddenly reducing the load on the grid due to less AC demand.

I have no written proof of this but have noticed exactly the same effect on my system, not just recently but for many years.

Well...i live a stones throw from the pacific coast and it has been raining steadily for 3 days now. So your theories would make sense. Pineapple express has swept in so it is also unusually humid as well. Bill Callahan's Dream River never sounded so good!!

When it rains, I notice my system has more liquidity in the midrange. ;)

It really is about the humidity in the air surrounding the equipment. Best is to always have it at about 45%, anything below or above that is not optimal for listening and conditioning your equipment environment.

I must say that here in Montreal the sound improve greatly because of the dampening of the snow outside you could really hear the difference almost like a line conditioner. So i like to listen to my system while there is a huge snow fall outside…..