Among a small shower of rain-related names, this pure version can have a cool, refreshing image.
Richard Pryor named his now-grown daughter Rain, and Rain is also one of the nature-named Phoenix family siblings. More recently, 24 actress Marisol Nichols named her daughter Rain India.
Rain also makes a nice nature middle name--Brooke Burke called her daughter Heaven Rain.
If you want to embellish this simply spelling (and alter the meaning), you might also consider Raine, Raina, Rainn, Rayne, Reine or Reign, the middle name of Willow Smith. Or you could take the Holly Madison route all the way to Rainbow.