Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
According to Kinbote (in VN’s novel Pale Fire, 1962, Shade’s mad commentator who imagines that he is Charles the Beloved, the last self-exiled king of Zembla), as many old glaziers, Gradus (Shade's murderer) could deduce rather accurately water temperature from certain indices of brilliancy and motion: