First of all, if the climate is too humid, you should not go outside
with any instrument.

There is a good reason to use wood for an outdoor organ, since wooden
pipes do stay in tune better under changing outdoor conditions,
especially temperature.

Glass and metal do react rapidly, with quite big changes in size,
so changing of temperature will make your organ sound awful.

In fact, the main reason for street organ builders to use wood for the
piping was (and is) the above.

Jan Kijlstra

 [ How does the temperature coefficient of expansion of wood compare
 [ with other materials?  Is the expansion of the pipe enough to make
 [ a difference?  Certainly the biggest effect of temperature upon
 [ the frequency of a flue pipe is the change of air density due to
 [ the air temperature change.  -- Robbie