It occasionally happens when the time comes to recite Birkat Ha値ebana that the moon is visible but covered by a thin layer of cloud. The Hida (Rav Haim Yosef David Azulai, 1724-1807), in his work Moreh Be弾sba (184), and the Ben Ish Hai (Rav Yosef Haim of Baghdad), rule that one should not recite Birkat Ha値ebana under such conditions. The Beracha should be recited only if the moon is completely exposed without any cloud cover at all, and thus even if there is a very thin cloud over the moon, the Beracha should not be recited. This is the ruling of Hacham Bension Abba Shaul (Israel, 1923-1998), who adds that if this happens on the last night when the Beracha may be recited, then one should recite the Beracha from a Gemara. There is an opinion of one of the Halachic authorities (Yaabetz) that one is permitted to verbally say Hashem痴 Name when reading a text from the Gemara containing G-d痴 Name. Therefore, in order to circumvent the issue of reciting Birkat Ha値ebana when the moon is covered by a thin cloud on the last night, one should read the passage of the Gemara in Masechet Sanhedrin which presents the text of Birkat Ha値ebana.
One should look at the moon before reciting Birkat Ha値ebana, and one should not look at the moon while reciting the Beracha.
Hacham Bension further noted that one may begin reciting Birkat Ha値ebana when the moon is clear even if he sees a cloud coming and anticipates the moon being covered by the time he finishes the Beracha. As long as one begins reciting the Beracha when the moon is clear, he may recite the Beracha even if the moon will be covered by the time one completes the recitation. Although some authorities disputed this ruling, one may rely on this view. (See Or Le痴ion, vol. 3, 4:3.)
Summary: One should not recite Birkat Ha値ebana unless the moon is completely exposed; even if the moon is visible but covered by a thin film of cloud, the Beracha should not be said. If the moon is visible but covered by a thin cloud on the final night when the Beracha can be said, it should be recited from the text of a Gemara (in Masechet Sanhedrin). One may begin reciting the Beracha when the moon is clear even if he sees a cloud that will cover the moon before he completes the Beracha.
