Each month, one is required to recite Birkat Ha'levana at any point from the seventh day after the Molad (the first appearance of the new moon) until the fifteenth of the month. One may not recite this Beracha unless he can actually see the moon; if clouds cover the moon, the Beracha may not be recited.
The question arises as to whether one may recite Birkat Ha'levana if the moon is covered by a thin layer of cloud but remains visible. Does the moon's visibility allow for the recitation of the Beracha, or does the layer of cloud cover require delaying the recitation of Birkat Ha'levana?
The Chid"a (Rabbi Chayim Yosef David Azulai, Israel, 1724-1806) in More Beshba, siman 184, , as well as the Ben Ish Chai (Rabbi Yosef Chayim of Baghdad, 1833-1909) in Parashat Vaykira 2, Halacha 23, ruled that one should not recite Birkat Ha'levana while a cloud covers the moon, even if the moon is still visible. This was indeed the practice of the students of Yeshivat Bet-El in Jerusalem. Some scholars extracted this position from an ambiguous comment of Rabbenu Yona Gerondi (Spain, 1180-1263), who wrote that one should not recite Birkat Ha'levana "Ad She'titbasem." Some interpreted this expression to mean that one should delay Birkat Ha'levana until Motza'ei Shabbat, when one has smelled the Besamim (spices). Others, however, explain that one should not recite the Beracha until the moon is "sweetened," meaning, until it appears clearly in the sky without any obstruction. Thus, if even a thin layer of cloud covers the moon, one should not recite Birkat Ha'levana until the cloud passes.
However, Hacham Ovadia Yoseph in Hilchot Chanukah, page 322, ruled leniently so long as one can benefit from the light of the moon enough to discern between 2 coins. (This is the opinion of the Peri Hadash, Bet Oved, and Mishna Berura in siman 426, seif kattan 3.)
Summary: The moon must be visible to allow for the recitation of Birkat Ha'levana. If the moon is visible but covered by a thin layer of cloud, one may recite the Beracha so long as he may benefit from the light of the moon.
