Seasonal pitch change:

[Ron Nossaman]

> 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


Percentage of breaking tension, more accurately, rather than 
tension.
Ron N