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