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Thin Cloud Covering over the Moon
Sephardic custom is to delay Kiddush Levanah if the moon is obscured by thin cloud cover (Ben Ish Chai op. cit. and Teshuvot Yabia Omer 5: Orach Chaim 24:2-7) since they prioritize reciting Kiddush Levanah at a more aesthetically pleasing sight than reciting it at the earliest opportunity. Ashkenazic Jews, though, seize the opportunity to recite Kiddush Levanah even if there is a thin cloud cover (Mishnah Berurah 426:3) due to their prioritizing Zerizim Makdimim L’Mitzvot.
Significantly, Rav Mordechai Lebhar (Magen Avot, Orach Chaim 426) writes that an exception should be made in Sephardic Kehillot, where some will not say Kiddush Levanah if they do not do so with the community on Motza’ei Shabbat. Moreover, when delayed until after Motza’ei Shabbat, Kiddush Levanah is often said under less ideal circumstances, including before it is completely dark and with a smaller crowd.
However, it is difficult to define thin cloud covering precisely. Rav Mordechai Willig, while acknowledging that it is a judgment call, offers the following guidance. One extreme is when the moon appears sharp, and the other is when it is vague, meaning there is moonlight, but one cannot discern the moon or its contours. Rav Willig regards thin covering as when one can fully see the moon, but it does not appear as sharp as it does on a very beautiful, clear night.
Significantly, Rav Shlomo Zalman Auerbach (Halichot Shlomo Rosh Chodesh 1: Dvar Halacha 44) permits Kiddush Levanah recital even if clouds partially cover the moon. The Eishel Avraham (of Buchatch; 2:1) is also inclined to rule thusly. Their rulings are supported by a report from the Leket Yosher (a student of the Terumat HaDeshen; p. 70) that he witnessed the Terumat HaDeshen say Kiddush Levanah even when clouds partially obscured the moon.
Interestingly, Rav Shlomo Zalman Auerbach (Halichot Shlomo, Rosh Chodesh 1 Dvar Halacha 46) thinks that one cannot say Kiddush Levanah if he sees the moon only through a telescope. He reasons that the Halacha does consider one to be seeing something (for ritual purposes) unless he sees it through natural means. Indeed, the Aruch HaShulchan (Yoreh Deah 84:36) famously writes regarding insects not visible to the naked eye that “if the eye cannot see something, it is as if it does not exist.”
Rav Chaim Sanzer’s Approach
The Mishnah Berurah (426:2) writes that if, after beginning to recite the Berachah, clouds covered the moon, one should nonetheless complete the Berachah. However, he rules that one should not begin to recite the Berachah if he believes that clouds will cover the moon before he completes the Berachah. In the Bi’ur Halacha (426:1 s.v. V’Nehenin), he notes that this ruling has a very solid basis - it stems from the Teshuvot Radbaz (1:341) and is cited by many Acharonim.
Interestingly, the Bi’ur Halacha (426:1 s.v. V’Nehenin) quotes Rav Chaim Sanzer, a great late nineteenth-century Chassidic Poseik who disagrees. He rules that if he saw the moon within Toch K’dei Dibbur (two to three seconds; the time it takes to say Shalom Alecha Rebbe), he may begin Kiddush Levanah even if the moon has already been covered!
The Sanzer Rav draws support from the Shulchan Aruch (Orach Chaim 227:3), which permits saying the Berachah on thunder and lightning Toch Kedei Dibbur of hearing these phenomena, even if when hearing it he was Halachically ineligible to say it.
Conclusion
Although Rav Chaim Sanzer is a Posek of great stature, the consensus does not accept his opinion. We should not follow Rav Chaim Sanzer’s leniency since Safek Berachot L’Hakel, we omit a Berachah in case of doubt. Rav Mordechai Willig agrees. However, the Piskei Teshuvot (426:8) rules that one may recite the Berachah even if he is unsure if the clouds will cover the moon before completing the Berachah. Rav Mordechai Willig agrees.
The Piskei Teshuvot infers his ruling from the Mishnah Berurah, which directs us to refrain from saying the Berachah if we assess that the moon will cover the moon before completing the Berachah. The Mishnah Berurah implies that one should refrain only if he believes that the clouds will envelop the moon, but not if he is unsure.
We add a lenient consideration in support of this ruling. The Chayei Adam rules (118:13) that although one should not begin the Berachah if he assesses that the moon will be covered by its completion, B’di’eved, he should finish it if he started to recite it. The Chayei Adam considers Rav Chaim Sanzer’s view in the case of B’dieved, implying that, in the event of uncertainty, we may rely on it Lechatchilah.