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If the moon is covered by clouds may one recite Kiddush Levana?
If the moon is covered by clouds, the blessing may only be said if the clouds are thin enough to still be able to benefit from the moonlight, meaning that one is able to recognize [majority of] matters which are recognizable due to moonlight. [If only part of the moon is coeverd by clouds, the blessing may be said.]
Bedieved: If one said the blessing while the moon was covered to the point that one could not receive benefit from its light, he must repeat the blessing at another time when the moon shines.
 M”A 426:1 from Radbaz 1:154; M”B 426:3; Shaar Hakolel 33:1; See Ashel Avraham Butchach 426
Other opinions and Sefaradi custom: Some Poskim rule one is never to say the blessing when the moon is covered even by a thin cloud. [Ben Ish Chaiy Vayikra 23; Chida in Moreh Bietzba 184, brought in Kaf Hachaim 426:18; See Halacha 5 for a story with Reb Hilel Paritcher who waited for there to be no clouds] Practically, so is the custom of the Sefaradim. Other Poskim rule one may recite Kiddush Levana even if one cannot benefit from the moonlight due to it being covered by clouds, so long as one can see its figure and outline of the moon. [Daas Torah 426]
 Ashel Avraham Butchach 426
 Leket Yosher p. 70
 Magen Avraham ibid