Seasonal pitch change:

[David Love]

The lower the tension in a given string the greater the change in pitch for
a given change in length.  Since the lowest tension tends to be in the low
tenor, that is where the largest change in pitch generally is.  The bass
section is usually the highest string tension and therefore changes the
least.  On many pianos the scale tensions also drop in the upper treble.
Scales that are designed with more equal tension through the piano will tend
to go out of tune more evenly.  


David Love
davidlovepianos at comcast.net 
www.davidlovepianos.com


What this does not explain tho... is why pitch change happens so 
unevenly across the scale as a whole.  Why do the lowest notes in the 
middle sections seem to have the largest reaction to pitch change, why 
does this pattern more or less reverse itself in the highest section, 
and why does the bass seem much less affected as a whole, but the 
highest notes are the ones most affected there ? I keep getting pointed 
back to something about the two planes (string and bridge surface) 
changing as a whole in relation to each other.

Cheers
RicB