Both the Piano Technicians Guild and the Steinway Web Sites discuss
relative humidity.  Quoting from http://www.ptg.org/care.html -

  "A piano functions best under fairly consistent conditions which are
   neither too wet or dry, optimally at a temperature of 68 degrees F
   and 42 percent relative humidity."

And quoting from http://www.steinway.com/html/tech/caring_living.html -

  "Ideally, your Steinway piano should reside in a temperate atmosphere
   where relative humidity ranges from 45% to 50%.  If this is not
   possible remember that the idea is to keep environmental factors
   within a fairly narrow range."

Joe Orens