Frasier and Niles compare how many times they have been the subject of "transference" (when a patient falls in love with their psychiatrist). Frasier boasts that he's had thirteen such patients; however in an earlier episode, he laments that he's never had a patient fall for him.
When Ronnie "arrives" while her mother is distracted in the kitchen, Frasier pour a vase of water over her head, claiming it's pouring down rain outside. However, Ronnie's mother arrived just a few minutes earlier not appearing wet at all and with no umbrella or head covering.
Even if it was raining Frasier is on the 19th floor thus Ronnee wouldn't still be as wet as the vase water made her appear.