Seasonal pitch change:

[Ron Nossaman]

> At the risk of sounding stupid, I am compelled to ask again: Does the 
> cast-iron plate change its dimensions with temperature changes? The 
> distance from tuning to hitch pins might change just as railroad tracks 
> shrink and swell in length - no? I'm just thinking it must be a 
> contributing factor.
> 
> Jason

Yes, it does. In the short term, temperature changes show 
nearly immediate pitch changes as the strings grow or shrink. 
The strings change quickly because they're of small diameter 
and low thermal mass. The thermal mass of the plate is HUGELY 
greater, so it responds much more slowly to temperature 
changes. The fun part is that the steel strings and the gray 
iron plate have a similar thermal expansion coefficient, so 
after the plate reactions catch up with the strings', the 
tuning isn't changed all that much.

Seasonal pitch swings are primarily wood and moisture related, 
rather than temperature related.
Ron N