Better when it rains?

I am noticing my tube system sounds especially wonderful when it is raining outside. Is this my imagination??
No, it is not your imagination. Your system is singing in the rain, just singing in the rain! What a glorious sound, you're happy again!
Seriously, back in the day, I noticed a deterioration of sound as humidity decreased. I don't doubt what you are experiencing is something similar.
Your system may sound better to you because of the added background noise the rain provides. For myself, I can't listen in a completely silent room. It just doesn't "feel" right. I usually have a small fan going on a very low setting. Just that little bit of background noise gives a more relaxed feeling to the room and system. I'm not sure why, but it works.
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.
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 outsideĀ…..
I suspect most folks would actually find the sound considerable better when a high pressure system (sunny and clear) is present and considerably worse when a low pressure system (stormy) is present. Depending on time of day, of course. :-)
I remember several years ago my system sounding unusually bland and undynamic after a prolonged drought. I suspected the electrical ground had deteriorated due to the dessicated soil's lower conductivity.

When the weather finally broke and the ground got a good drenching, the sytem came back to life.

Now, when I water the lawn and flower beds I also water my ground stakes. The neighbors think I'm nuts.
I've noticed the same thing. Perhaps the additional humidity affects the material in speaker diaphragms in a pleasing way?
great idea of white noise generator.
humidity notes are also very big factor especially for vinyl playback will be beneficial.