I think it has something to do with Mr. and Mrs. Weston’s secret wishes for Emma and Frank:

Part of her meaning was to conceal some favourite thoughts of her own and Mr. Weston's on the subject, as much as possible. There were wishes at Randalls respecting Emma's destiny, but it was not desirable to have them suspected; and the quiet transition which Mr. Knightley soon afterwards made to "What does Weston think of the weather; shall we have rain?" convinced her that he had nothing more to say or surmise about Hartfield. (Chapter 5)