Barb Schmidt's excellent twainquotes.com web site suggests that the quote about San Francisco should not be considered authentic, pending an authoritative source being found. There is a link there to a similar description of Paris, which is a bit more elaborate than the quote to which you refer. It describes a horrible winter in Paris and speaks of a summer by comparison. I've not yet come across anything I would consider definitive in attributing the San Francisco line to M.T. Roger Durrett