For the home weather station of store no.15, what was the dew point on 2012/2/18?
store no. 15 refers to store_nbr = 15; on 2012/2/18 refers to date = '2012-02-18'
SELECT T1.dewpoint FROM weather AS T1 INNER JOIN relation AS T2 ON T1.station_nbr = T2.station_nbr WHERE T2.store_nbr = 15 AND T1.`date` = '2012-02-18'