What is the science behind basses falling out of tune from weather?

[PostBop]

I have a Shen hybrid, its very new (born on date is 3/11). When the temps and humidity soared yesterday the bass fell out of tune. I'm assuming the wood swelled and the strings loosed. Today it is a beautiful mild day. The bass is tuned a half step lower.

Is my assumption about swelling correct? Are there things to avoid or be careful of when retuning? If I know it is going to be hot and humid, is detuning the bass a good preventative step? Assume A/C is not always an option.

Thanks

[ZUEG]

don't worry about detuning or anything. humidity is an enemy of wooden instruments. the best you can do is to try and control the environment that your bass is stored in. the proper humidity should be around 50% plus or minus. you'll have to tune every time you play anyhow. even in a controlled environment tuning changes. Marty

[Phendyr_Loon]

Nothing will break on your bass during retuning as a result of hot and/or humid weather.
However, if the temperature is like 10 degrees, you are likely to break something if you start messing with the tuners.
Your bass doesn't like extreme temperatures just like you don't. Warmer weather is better than cold.

[Eric Hochberg]

Originally Posted by PostBop

I'm assuming the wood swelled and the strings loosed. Today it is a beautiful mild day. The bass is tuned a half step lower.

The top swells in higher humidity and the pitch raises. Pitch lowers when the top "shrinks". The bass will have to acclimate to NJ weather, expecially if it's a lot different than the weather in China where the bass was made. Just make sure that next winter you keep humidity around 40% where the bass lives most of the time. Also, have the soundpost checked. You may need a shorter one for winter.

[Jerry Callo]

Actually, a little humidity is good for wood. It's dry heat that is most damaging.