Please read Alexey Sklyarenko's annotations on Pale Fire, Ada and other Nabokov works here.
Upon his arrival at Ardis, Daniel Veen (in VN’s novel Ada, 1969, the husband of Van’s, Ada’s and Lucette’s mother Marina) tells Van that it is going to rain in a few minutes, because it started to rain at Ladore, and the rain takes about half-an-hour to reach Ardis: