Bracha upon Seeing a Rainbow
Upon seeing a rainbow, one should make the specific bracha that the rabbis formulated for this special occasion. The text of the bracha on upon seeing a rainbow is: ברוך אתה ה' אלוקינו מלך העולם זוכר הברית, (ו)נאמן בבריתו, וקיים במאמרו. The transliterated text is: Baruch Atta Hashem Elokenu Melech HaOlam Zocher HaBrit, (Ve)Neeman Bivrito, VeKayam BeMaamaro.  Some opinions hold that the beracha should not be made with Hashem's name. 
- One can make this bracha one time for every rainfall and not again until the rainbow has cleared up completely and then it rains again. 
- Some say that one must see the entire rainbow in order to make the bracha.  Others, however, hold that there is what to rely on if one makes the bracha upon seeing a part of the rainbow.
- One should not stare at the rainbow, rather one should look at it briefly and then make the bracha. 
- One should not tell one’s friend about the rainbow even if one does it in order that one’s friend is able to make the bracha, as it’s similar to spreading bad news. Similarly, one shouldn’t make the bracha loudly so that one’s friend hears it and recognizes the rainbow. However, others assume that since it is a mitzvah to recite this bracha one should tell others about the rainbow.
- If one sees the rainbow at night or through glass one may recite the bracha. 
- Some say that if an individual is told that there is a rainbow visible during chazarat hashatz (after the bracha of hakel hakadosh), they should leave and make the bracha. Others write that this is certainly not required.
- Gemara Brachot 59a, Shulchan Aruch O.C. 229:1, Mishna Brurah 229:3 writes that the Tur and Rambam (Brachot 10:16) add a vav before Neeman. Vezot HaBracha (pg 156) codifies the text of the Mishna Brurah with a vav before Neeman. Halacha Brurah 229:1 and Aruch HaShulchan 229:1 follow the text of shulchan aruch. The Kitzur Shulchan Aruch 60:4 includes the vav. Although the Bet Yosef 229:1 adds that "everyone" agrees that there is no conclusion to this bracha (unlike longer brachot which have a concluding bracha), the Elya Rabbah (Orach Chaim 229:1) notes that he must only be referring to the authorities that he cited earlier in that section, since other rishonim did have a concluding blessing here. The Rambam (Brachot 10:1-16) appears to include this bracha as a bracha of praise to Hashem.
- Bach (Orach Chaim 229:1) cites the Ra'avad as having this opinion, but notes that it has been rejected from halacha. Ben Ish Chai (Ekev, 17) cites Rabbi Yonatan Eybeschutz as explaining that there are two types of rainbows. Although the Ben Ish Chai concedes that if one were to apply Rabbi Eybeschutz's opinion, it would be better to say the beracha without mentioning G-d's name (and kingship), he staunchly defends the general custom to say G-d's name. In his conclusion, he notes that if one wants to be stringent and only concentrate on the name, but not mention it outright, "they should not be rejected".
- Mishna Brurah 229:2 writes that even within 30 days one can make the Bracha again upon seeing another rainbow similar to the laws of seeing lightning and hearing thunder where the original sight has ended. BeYitzchak Yikra 229:2 Rav Nevinsal writes that the number 30 used by the Mishna Brurah wasn’t specific since it’s possible to make the Bracha more than once in a day. [See Vezot HaBracha (pg 156, chapter 17) who quotes Mishna Brurah as saying that one can make another Bracha as long as one removed one’s mind from the rainbow. However, the language of Mishna Brurah implies that the rainbow must clear up before one can make another Bracha.]
- Beiur Halacha 229:1 s.v. HaRoeh writes that it’s unclear whether one can make the Bracha for seeing a part of the rainbow or only if one saw the entire semicircle crescent. Teshuvot Vehanhagot 3:76 concludes that therefore, one may not make the blessing on anything less than a full semicircle rainbow. Vezot HaBracha (pg 156, chapter 17) writes that Rav Elyashiv rules that if one saw the entire semicircle one can make the Bracha even if it’s missing a piece.
- Chazon Ovadia (Brachot p. 473), Halacha Brurah 229:2, Birkat Hashem (v. 4, 4:35)
- Shulchan Aruch 229:1, Mishna Brurah 229:5, Aruch HaShulchan 229:2
- Mishnah Berurah 229:1, Citing Chayei Adam 63:4.
- Rav Nebenzahl in BeYitzchak Yikarei 229:1, although Rav Zilberstein (Chashukei Chemed to Berachot 59a) permits hinting to others by asking them what the words of the beracha are.
- Rabbi Mansour on Dailyhalacha.com, Brit Kehunah Ma'arechet Kuf, Ot Gimmel, Rabbi Ovadiah Yosef (cited in Yalkut Yosef Orach Chaim 229 footnote 1) and Rabbi Eliezer Melamed (Peninei Halacha Berachot 15:10).
- Halacha Brurah 229:3
- Sefer Chassidim 807., codified by Elya Rabbah Orach Chaim 229:1.
- Piskei Teshuvot Orach Chaim 229:3.