What is the maximum humidity in Powell Street BART when bike 496 was borrowed from the station on 8/29/2013?
Powell Street refers to start_station_name; bike 496 refers to bike_id = '496'; start_date = '8/29/2013';
SELECT T2.max_humidity FROM trip AS T1 INNER JOIN weather AS T2 ON T2.zip_code = T1.zip_code WHERE T1.start_date LIKE '8/29/2013%' AND T1.bike_id = 496 AND T1.start_station_name = 'Powell Street BART'