I've discovered and proven that without a doubt, air, specifically the chemical composition of air in the immediate vicinity of a bass will drastically alter it's tone.
The air in our atmosphere is primarily made up of Nitrogen, Oxygen, Argon, Carbon Dioxide and cat hair. That, however, varies depending on exactly where you are. The CO2 content will be higher in the chimney of a factory than the top of Mt. Everest. The cat hair content will be higher in a pet shelter than in your freezer. But, until now no one has actually attempted to see how those differences could affect tone!
For the first test I left my car running in the garage and played through my Fender FMT through my Markbass/Bergantino rig. Admittedly, this wasn't the safest method in the world but I think mitigated most harmful effects by tying a tshirt across my face. For the second, I took my rig to a hotel and played on the roof. Granted these were only two environments but I think the results were significant enough to leave it there. In the garage my tone almost sounded like it was choking. It had a very muted high end and the upper midrange sounded strained. Really, only the lows and low-mids were present which made for a very tumpy sound without much definition. I would have continued my test but I think I passed out. On top of the hotel my tone sound much more open across all frequencies. Nothing was particularly emphasized or defeated. It sounded like a much more accurate representation of the bass itself.
I hope this helps you all in decided where you play your gigs and what to expect of your tone in different atmospheric conditions.