How many samples of clouds are there in the image no.2315533?
samples of clouds refer to IMG_ID where OBJ_CLASS = 'cloud'; image no.2315533 refers to IMG_ID = 2315533;
SELECT SUM(CASE WHEN T1.IMG_ID = 2315533 THEN 1 ELSE 0 END) FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T2.OBJ_CLASS = 'clouds'