Hashem spoke to Bnei Yisrael from amidst a pillar cloud. Rashi says that this applies equally to Shmuel HaNavi. Hashem spoke to Shmuel through a cloud, as He did to Moshe Rabbeinu and Aharon HaKohen. Ibn Ezra disagrees, contending that this applied only to Moshe and Aharon. In Shmuel (I,9,10-12) He relates that, when Shaul HaMelech came searching for Shmuel, a group of young girls pointed him in the direction of the prophet’s house. It was obvious that this was Shmuel’s house, because a pillar of cloud in an otherwise cloudless sky hovered over Shmuel’s doorway.
Rav Sadya Gaon points out that, when Hashem speaks to a prophet, the entire sky is clear- not a cloud to be seen- except for the pillar of cloud hovering in front of the Navi. This is his sign that he is about to hear a true prophecy.
