If one forgot to say Mashiv Haruach:[1]

Nusach Arizal/Sefarad-Said Morid Hatal:[2]

One who [Davens Nusach Arizal or Sefarad, and] during the rainy season [accidently] mentioned Morid Hatal [as is normally said in the summer months] instead of Morid Hageshem, does not need to repeat [the blessing, or Shemoneh Esrei].[3]

Remembered before concluding the blessing of Mechayeh Hameisim:[4] The above law applies even if he did not yet complete the blessing[5] [of Mechayeh Meisim, nevertheless he is not to retract to recite Morid Hageshem, and is rather to continue with the conclusion of Mechayeh Hameisim]. [He is not even to say it in the area that he remembered and continue from there.[6]]

Nusach Ashkenaz-Did not say Morid Hatal:[7]

One who [Davens Nusach Ashkenaz and thus] does not recite Morid Hatal [in the summer months], if Morid Hageshem was omitted in the rain season [i.e. from Musaf of Shemini Atzeres], then he must repeat Shemoneh Esrei.[8] This applies even if he recited the words Mashiv Haruach, but omitted the words Morid Hageshem [or Morid Hatal].[9] Remembered right away:[10] One is only required to return to the beginning of Shemoneh Esrei if he already concluded the blessing of Mechayeh Meisim and began saying the next blessing [even the first word of “Ata”] One must repeat from the beginning irrelevant to where in the prayer they remembered their omission. Similarly, if one remembered only after concluding the entire prayer, he must repeat the Davening. If, however, one remembered prior to saying Hashem’s name in the blessing of Mechayeh Meisim, then he is to say Morid Hageshem in the place that he remembers and continue from where he left off. [If he already said Hashem’s name, then he is to finish the blessing and say Morid Hageshem prior to beginning the words of the blessing of Ata Kadosh. The same applies] if he only remembered after concluding the blessing of Mechayeh Meisim but before beginning the next blessing [even the first word of Ata], then he is to say it there. If, however, he already began even the first word of the next blessing he must return to the beginning of the prayer, as explained above.

If one is in doubt if he said Mashiv Haruach:[11] If one who Davens Nusach Ashkenaz is in doubt if they mentioned Morid Hageshem, then for the first 30 days after Shemini Atzeres [until 22/23[12] Kisleiv] we assume it was omitted, and hence follow sthe same ruling as one who omitted it.[13]

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[1] Admur 114:6-7

[2] Admur 114:6

[3] Admur ibid; Michaber 114:5; Rambam Tefila 10:8; Rif Taanis; Tosafus Brachos 29b; Rosh Brachos 4:14; Yerushalmi beginning of Taanis

The reason: As although the dew [i.e. Tal] which he mentioned never stops [and is hence not in need of his prayers, unlike rain], nevertheless, mentioning it is considered a praise to Hashem just as is the mention of rain, as just as rain gives life to the world so too the dew gives life to the world, as through it the produce is blessed. (In fact, in a number of instances, the Sages referred to dew as the dew of resurrection. It is for this reason that there are people who mention the praise of dew throughout all the days of the summer in order so if one forgets to say Morid Hageshem during the rainy season, they retain a Chazakah [status quo] that Morid Hatal was recited, as they are accustomed to say in the summer, and thus they do not need to repeat Shemoneh Esrei.) [Admur ibid, parentheses in original; M”A 114:7; Taz 114:11; Beis Yosef 114]

[4] Admur ibid in parentheses; Implication of Beis Yosef 114 as brought in P”M 114 A”A 7; See Chikrei Halachos 7/11 that this matter is in truth unclear from the Poskim, and hence Admur placed it in parentheses.

Other opinions: Some Poskim write that according to the Michaber one is required to go back so long as he did not finish the blessing. [P”M ibid]

[5] Meaning even he did not yet say Hashem’s name in the concluding blessing.

[6] Implication of Admur ibid as evident with the contrast ruling in Admur 114:7 for Nusach Ashkenaz

[7] Admur 114:6

[8] Admur 114:6; Michaber 114:5; Brachos 29a; Yerushalmi beginning of Taanis; Poskim in previous footnotes

[9] Admur ibid in parentheses

The reason: Although the world cannot sustain itself without wind, nonetheless, it is not considered giving life to the world as is the rain and dew [and hence one does not fulfill his obligation by mentioning wind instead of rain or dew]. [Admur ibid in parentheses]

[10] Admur 114:7; Michaber 114:6;

[11] Admur 114:10; Michaber 114:8

[12] This depends on if we count Shemini Atzeres as the 1^st day or not. Vezatrcuh Iyun.

[13] Prior to the conclusion of the 30 days one is not to repeat the beginning of the blessing of Michayeh Meism until … Morid Hageshem for a total of 90x in order to make for oneself a status quo that he has now regulated himself to say it and hence no longer needs to go back in case of doubt, as there are opinions who argue on this ruling and hence one ends up entering himself into a dispute if he does not remember if he said it prior to the 30 days concluding.