The Beracha of Birkat Ha’hama, which will be recited this year (5769/2009) on Ereb Pesah, may be recited only when one actually views the sun. If clouds cover the sun, one may not recite the Beracha until the sun becomes visible.
Some authorities address the interesting case where clouds covered the sun, but one nevertheless recited the Beracha, omitting "Shem U’malchut" (the phrase, "Hashem Elokenu Melech Ha’olam"). For example, the person recalled learning that when the sun is not visible on the day of Birkat Ha’hama, one may still recite the Beracha without "Shem U’malchut," and he therefore recited the Beracha in this fashion despite not being able to see the sun. If the sun subsequently appeared, should he recite the Beracha upon seeing the sun at that point (assuming that this occurred within three hours of the day, after which point the Beracha can no longer be recited)? Or, do we consider this individual has having already recited the Beracha, such that he may no longer recite it?
Rav Pinhas Zevihi (contemporary), in his work Ateret Paz (p. 224), rules that one may repeat the Beracha in such a case. There is a Halachic principle that a Beracha recited without "Shem U’malchut" does not have the formal status of a Beracha, and therefore the person is not considered to have recited Birkat Ha’hama that day. As such, he may and should recite the Beracha when the sun becomes visible.
Rav Zevihi further writes that if a very cloudy day is expected, but the sun happens to be visible at sunrise, then one may recite the Beracha immediately at sunrise, even before praying Shaharit. Although generally it is preferable to recite Shaharit before reciting Birkat Ha’hama, one may reverse the sequence in this case due to the concern that he would otherwise miss the opportunity to recite Birkat Ha’hama.
If the sun is covered by clouds, but the sun briefly appears while one recites Shema or the Berachot preceding or following Shema, may he interrupt his Shema recitation to recite Birkat Ha’hama?
Rav Zevihi (ibid. p. 227; listen to audio recording for precise citation) writes that in such a case, one should preferably complete the paragraph or Beracha that he currently recites, and then recite Birkat Ha’hama. If, however, the sun appears only momentarily, and one is concerned that he may lose his opportunity to recite the Beracha if he delays the recitation even for a moment, then he may recite the Beracha at that point, even in the middle of a paragraph. Birkat Ha’hama is a "Misva Oberet" – a Misva confined to a very limited time frame – and therefore one may even interrupt Shema or the Berachot of Shema to fulfill this Misva. Rav Zevihi adds, however, that in such a case one should recite only the Beracha itself, and not the verses, Psalms and hymns traditionally recited before and after Birkat Ha’hama.
This is also the ruling of Hacham Ovadia Yosef, in his Yehave Da’at (vol. 4, 18:8).
Summary: One recites Birkat Ha’hama only upon seeing the sun; if clouds cover the sun, he may not recite the Beracha until the sun becomes visible. If a cloudy day is expected, one who sees the sun early in the morning may recite the Beracha even before reciting Shaharit. Likewise, if the sun momentarily appears as one recites Shema or the Berachot of Shema, he may interrupt his recitation for Birkat Ha’hama. Preferably, however, he should first complete the paragraph or Beracha he currently recites and then recite Birkat Ha’hama.
