turntable sounds better on warm summer nights

I second Chadnliz's observation. Also, do you listen LATER on summer nights, when the power grid is cleaner? Do you have electric heat in the winter? Can you check the voltage at the outlet that powers your system in both summer & then in winter?

There are many things in play here. Warm summer nights might mean that your speakers and electronics too are operating differently...your electricity and any other permutation could change the sound.

Does listening to and enjoying music play any part in your activities?

Did you see the other thread about getting a humidifier to improve sound quality?

Arthur

Could it be changed bearing tolerances due to dissimilar metals ? My rega has a brass bearing housing, with a steel shaft: these metals expand and contract differently. It's pretty common to have a brass or bronze housing with a steel shaft, and a steel, ceramic, or saphire bearing ... all different materials.

One of the Brinkmann turntables has a small heater in the bearing housing to maintain a constant temperature. It seems a bit extreme to me, but perhaps there is a measurable difference in performance.