Better when it rains?

The effectiveness of your home's ground is in part a function of basically having most, if not all, of the right chemical elements (some 80+ different ones) present in sufficient amounts in the soil where your ground rod has been driven. But, it's overall effectiveness for how well your system performs rises and falls as those chemicals are actually in the presence of moisture. That's why, ideally, the ground rod should be deep enough to penetrate the ground water level. The US spec is generally 8ft, but often that ends up being insufficient in many areas. Arid climes are more difficult to get good ground in than wet ones and, with good grounding conditions or otherwise, often systems sound better after a good, 2 or 3 day, gentle soaking rain. Regards. John

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.