Among the bike trips started on the days with a fog in 2013, how many of those trips started from the station "2nd at Townsend"?
started on the days with a fog refers to start_date where events = 'fog'; in 2013 refers to date LIKE '%2013'; started from station refers to start_station_name; start_station_name = '2nd at Townsend';
SELECT COUNT(T1.start_station_name) FROM trip AS T1 INNER JOIN weather AS T2 ON T2.zip_code = T1.zip_code WHERE T2.date LIKE '%2013%' AND T2.events = 'Fog' AND T1.start_station_name = '2nd at Townsend' AND T2.zip_code = 94107