Saying a blessing upon seeing a rainbow:
One who sees a rainbow is to say the blessing of “Baruch Ata Hashem Elokeinu Melech Haolam Zocher Habris Neman Bevriso Vikayam Bimamaro.” [Some Poskim, however, rule that the rainbow referenced to in this law is not necessarily the rainbow that we see today and hence one is to recite the blessing without Hashem’s name. Practically, the Poskim negate this opinion and rule the blessing is to be said with Hashem’s name upon seeing a rainbow. Nonetheless, one who avoids saying Hashem’s name and simply thinks it in his mind is not to be protested. The Chabad custom is to recite the blessing with Hashem’s name.]
Not to stare excessively at a rainbow: It is forbidden for one to stare excessively at a rainbow. [However, it is permitted to gaze at it without an intense stare or prolonged contemplation. The rainbow corresponds to the Shechina, and hence one who looks at it is considered to be looking at the Shechina. The Talmud states that one who looks at it excessively is considered to not care for the honor of His Maker, and it is thus befitting for him to not have come into the world. Likewise, one who does so causes his eyesight to weaken. The Zohar states that there are Kelipos which surround the rainbow and try to darken its light, thus explaining another reason for why it should not be looked it.]
Not to tell others of the rainbow: One who sees a rainbow is not to inform others of it, as doing so is considered slander. [If one was informed of its presence, he has no obligation to go see it in order to recite the blessing.]
Q&A on the blessing
How much of the rainbow must one see in order to say the blessing?
Some Poskim question whether one is required to see the entire bow of the rainbow, which is half of a circle, or seeing even part of it suffices. [Practically, one may say the blessing even if he does not see the entire bow of the rainbow. However, some Poskim conclude that it is not to be said.]
Is one to stand upon saying the blessing?
There is no requirement to stand upon saying the blessing, although many are accustomed to do so, as with any blessing of praise to Hashem.
If one did not say the blessing right away upon seeing the rainbow, may it still be said?
The blessing may be recited so long as the rainbow is still visible, even if one did not say it at the first glance.
How often is the blessing over a rainbow to be recited?
The blessing is to be repeated each time one sees a rainbow, even within thirty days from a previous sighting, similar to the law which requires one to repeat a blessing for another lightning or thunder [of another storm]. [However, in a single rainfall, or storm clouds, which creates the reflection of a rainbow, one does not repeat the blessing upon seeing multiple rainbows, unless the clouds cleared up and a new rain/clouds arrived, just as we rule regarding thunder and lightning. Nonetheless, the blessing is to be repeated upon seeing another rainbow the next day even if it is from the same clouds/rain, just as we rule regarding thunder and lightning.]
What is the law if one sees a rainbow in the midst of Davening?
This follows the same law as one who sees lightning or hears thunder, of which we rule that the blessing is to be said so long as one is prior to Shemoneh Esrei.
Q&A on the staring
May one take a picture of a rainbow?
Seemingly, one is to avoid taking a picture of a rainbow, as it is equal to staring at it for the sake of study, which is forbidden to be done. Vetzaruch Iyun.
Is one to avoid staring at the reflection of a rainbow seen within water?
Some write that one is to avoid doing so.
The message of the rainbow:
The purpose of the rainbow is to arouse Divine mercy upon the world and grant it protection. The Zohar states that when the rainbow is seen, it is a sign that the Reshaim have reached the point of needing destruction, and the forefathers Daven on their behalf to save them.
How can the rainbow serve as a message from G-d if it is embedded within nature?
Several answers have been offered by the Mefarshim:
1. There are two types of rainbows, one being the natural rainbow and the second being the rainbow which Hashem placed as a sign for the covenant.
2. Hashem could arrange for it to rain only at night, hence causing the rainbow to never be seen.
3. Hashem to begin with planted the rainbow within nature because he was aware of the covenant that he would make.
 Michaber 229:1; Brachos 59b
Ruling of Admur: Admur omitted the law of a rainbow from Seder and Luach Birchas Hanehnin. The Rebbe suggests that perhaps the reason is because it was uncommon to see rainbows in Russia [Igros Kodesh 16:328; Likkutei Sichos 10:200; Sichas Bechukosaiy 5741 3:468; printed in Shulchan Menachem 1:131]
 Background of Nussach and not to say the sentence of Neman prior to the blessing: In Brachos ibid it states that one is to state the blessing of Baruch Zocheir Habris, and then brings a Braisa which states that one is to say Neman Bevriso. Rav Papa concludes that one is to say both. Based on this Gemara, it became a widespread custom for people to say the sentence of Zocher Habris prior to the blessing of Neman Bevriso, and not included within it as part of the blessing. This in truth is a mistake, as one must include it in the blessing as writes Michaber ibid. [Taz 229:1; M”A 229:1; Kneses Hagedola 229; Kitzur Shlah p. 27; Machatzis Hashekel ibid; Kaf Hachaim 229:3]
Nussach of Vineman: Some write the Nussach of Neman with a Vav and read Vineman. [Brachos ibid; Tur 229; Rambam; Kneses Hagedola 229; M”B 229:3; Birchas Habayis 29:19; Kaf Hachaim 229:3; Piskeiy Teshuvos 229:1 footnote 1] Others rule it is to be said without a Vav. [Nussach in Michaber ibid; Biur Hgr”a 229; Levush 229] Practically, the Nussach in many Siddurim is printed with a Vav. [See Siddur Tehilas Hashem]
 Rav Yonason Eibashitz in Yearos Devash 1 Derush 12 that there are two types of rainbows, one being the natural rainbow and the second being the rainbow which Hashem placed as a sign for the covenant, and therefore [since we do not know which is which] one is to recite the blessing without Shem Umlachus
 Ben Ish Chaiy Eikev 17; Kaf Hachaim 229:4; Piskeiy Teshuvos 229:2; Many Mefarshim explain how the sign of Hashem and His covenant is apparent even within the natural phenomenon of the rainbow we see today, hence removing the basis for the ruling of the Yearos Devash. [See Ramban Noach; Shlah Hakadosh Parshas Noach that he heard from the Rama that Hashem could make it rain only by night and hence cause a rainbow to never be seen]
 The reason: As all the Poskim record the ruling that a blessing is to be said and do not differentiate in this matter, and so is the custom of all Israel, and hence we cannot uproot this custom simply due to words said in the form of Derush. [Ben Ish Chaiy ibid]
 Ben Ish Chaiy ibid; Kaf Hachaim ibid
 Hayom Yom 29th Tishreiy; Igros Kodesh 16:328, printed in Shulchan Menachem 1:352, that so was the directive of the Rebbe Rayatz; See Likkutei Sichos 10:200; Sichas Bechukosaiy 5741 3:468; printed in Shulchan Menachem 1:352
 Michaber ibid; Chagiga 16a as explains Rosh and Tur
 Beis Yosef 229 in name of Rosh “The prohibition is only to stare as one who stares focuses on all the details in a much greater fashion than one who simply looks; Biur Hagr”a “One must see it in order to say the blessing and Lashon Histaklus means with great contemplation”; Machatzis Hashekel 229:1; Olas Tamid 229:1; Kaf Hachaim 229:5
 Zohar Beshalach 66b
 Rebbe Aba in Chagigah ibid; Machatzis Hashekel ibid
 M”A 229:2; Chagigah 16a [see Machatzis Hashekel ibid and P”M 229 A”A 2 who replace the word Shlah in M”A ibid]; Olas Tamid 229:1; M”B 229:5; Kaf Hachaim 229:5
 Tikkunei Zohar Tikkun 18 p. 36b
 Chayeh Adam 63; Pischeiy Teshuvah; M”B 229:1; Kaf Hachaim 229:1
 Makor Chaim Kitzur Halachos 229; Piskeiy Teshuvos 229:3
 Biur Halacha 229:1; “Haroeh”
 Implication of Setimas Haposkim who do not mention such a requirement
 Teshuvos Vihanhagos 3:76; Piskeiy Teshuvos 229:1
 Piskeiy Teshuvos 229:1; 218:1; See Siddur Yaavetz that one is to say all the morning blessings in a standing position due to it being a blessing of praise, and so is the custom amongst Ashkenazi Jewry; See however Kaf Hachaim [Falagi] 9:7 and Yechaveh Daas 5:4 who write that Sephardic Jewry is accustomed to sit while reciting the blessings in order to increase concentration.
Background: From the letter of the law, one is only required to stand when saying blessings which involve commands, such as when saying the blessing over Tzitzis, and Tefillin. [Admur 8:3] However, blessings of praise do not require standing. [P”M Pesicha Brachos 18; However, see 432 M”Z 1 who leaves this matter in question] However, the Yaavetz ibid writes that all blessings of praise of Hashem are to be said standing. See Piskeiy Teshuvos 218:1
 Piskeiy Teshuvos 229:1
 Machazik Bracha 229:1; Shaareiy Teshuvah 229:1; M”B 229:2; Kaf Hachaim 229:2; See regarding thunder and lightning: Seder 13:16; Luach 12:25; Michaber 227:2; Piskeiy Teshuvos 227:7; Halacha Berura [Yosef] 227
Other opinions: Some Poskim rule that the blessing over the rainbow is to only be said one time in 30 days. [Opinion brought, and negated, in Machazik Bracha ibid]
 Birchas Habayis 29:19; Piskeiy Teshuvos 229:1; See regarding thunder and lightning: Admur and Poskim ibid
Other opinions: Some Poskim imply from the Shaareiy Tehsuvah ibid that the blessing is to be repeated each time one sees a new rainbow, even during the same storm. [Ketzos Hashulchan 66 footnote 26] Vetzaruch Iyun Gadol, as the Shaareiy Teshuvah clearly states in his words “That have ended” which refers to the dispersing of the clouds.
 Birchas Habayis 29:19; Piskeiy Teshuvos 229:1; See regarding thunder and lightning: Mamar Mordechai 227:3; Birchas Habayis 30:5; M”B 227:8 based on Yerushalmi; Kaf Hachaim 227:12; Ketzos Hashulchan 66 footnote 23; Piskeiy Teshuvos ibid; Halacha Berurah ibid
Other opinions: From some Poskim it is implied that there is no difference between that day and the next day, and also the next day a second blessing may not be said if the clouds of the previous day’s storm have not yet dispersed. [See regarding thunder and lightning: Implication of Setimas Haposkim of Admur ibid; Michaber ibid; Tur 227 who all record the ruling of the Yerushalmi but omit its ruling that the next day a blessing may be recited; brought in Ketzos Hashulchan 66 footnote 23; Gedulas Elisha 227:13]
 Piskeiy Teshuvos 229:1; See regarding thunder and lightning: Admur 66:4 [even in midst of a Perek]; M”A 66:5 [like Admur]; Kitzur SHU”A 16:3; M”B 66:19 [brings dispute]; Kaf Hachaim 66:16; Ketzos Hashulchan 19:1; Piskeiy Teshuvos 227:5 [bets to only say between Perakim]
 Yalkut Hagershoni 229:2; Ruach Chaim 229; Piskeiy Teshuvos 229:2
 See Piskeiy Teshuvos 229:2 footnote 9
 Rav Yonason Eibashitz in Yearos Devash 1 Derush 12
 Shlah Parshas Noach in name of Rama
 Shlah Parshas Noach