The Chachamim postponed the commencement of the request for rain in Babylonia (Bavel) until the sixtieth day of the winter season (which comes out to be the 4th or 5th of December). This is because there is an abundance of water from the Tigris and Euphrates Rivers, and therefore it is unnecessary to request rain in that area at the very beginning of winter. All people living outside of Israel follow the custom of Babylonia, and start asking for rain starting from the sixtieth day of the season (Shulchan Aruch 117:1).
Regarding a person residing in Israel who leaves the country for a few months, there is disagreement. Some say that he should request rain as they do in Israel, for that is his home (Pri Chadash). Others say that he should request rain according to the custom of the place where he is (Birkei Yosef). In any situation of uncertainty, one should request rain in the berachah of Shome’a Tefillah and not in Birkat HaShanim, thereby fulfilling his obligation according to all opinions. See the note for details of the halachah.
In places that require rain in the spring, the request for rain in Birkat HaShanim should not be continued after Pesach. Instead, the law concerning those places resembles the law of individuals; the people in those areas pray for rain in Birkat Shome’a Tefillah in which every person may add his own personal requests (Shulchan Aruch 117:2).
People living in countries south of the equator, such as Argentina, Brazil, and Australia, also request rain when it is winter in Israel. Despite the fact that it is summer in those countries at that time, since Israel is the center of the Jewish world, Jews everywhere follow its custom and make the request for rain according to winter in Israel.
However, in places where the rain causes damage in the summer, the people do not practice the minhag of Israel, so as not to request something that is harmful for them. Instead, all year round they recite the wording of the summer in Birkat HaShanim, and request rain in Shome’a Tefillah. During their winter they should have in mind the place in which they live, and during Israel’s winter they should think of Israel.
One traveling from Israel, or the northern countries to visit those areas should continue requesting rain according to the winter of Israel, even if the rain causes damage there (She’arim Metzuyanim BaHalachah 19:3).
. In the countries closest to Israel, in which the climates are dry and more water is needed, it is customary to begin to request rain at the same time people in Israel do, on the eve of the seventh of Cheshvan. That is the custom even in far-away countries where the climate is similar to that of Israel, such as Djerba (Yalkut Yosef 117:4).
. The dispute of the Acharonim is explained in the Mishnah Berurah 117:5. Kaf HaChaim 11 tends to agree with the opinion of the Birkei Yosef who maintains that one requests rain as they do in the place where he is at that time. Many mention the advice that one should make his request in Shome’a Tefillah (Tefillah KeHilchatah p. 235; Yalkut Yosef 117:15; Ishei Yisrael 23:37). In that way, he fulfills his obligation according to all, since even when there is an obligation to ask for rain, the request can be made up in Shome’a Tefillah if missed, and on the other hand, even if it is not the proper time to make such a request, if he asked for rain in Shome’a Tefillah, he need not repeat his prayer.Details of certain laws: An Israeli who leaves the country: 1) If he leaves before the seventh of Cheshvan, he asks for rain in Shome’a Tefillah starting from the seventh of Cheshvan. 2) If he leaves after the seventh of Cheshvan, since he already began requesting rain, he continues to do so in Birkat HaShanim (Kaf HaChaim 13, in the name of Kesher Shel Gudal). 3) If he leaves with his family for more than a year, he is considered at that time to be a resident of a different country and he follows the custom there.
A person from a different country who comes to Israel: it is better that he practices like those in Israel, and therefore if he intends to return to his country after the day on which it is necessary to start requesting rain there, he requests rain as those in Israel do. If he intends to return before the day that it is necessary to start requesting rain there, some say he makes his request in Shome’a Tefillah. Others say that he requests as they do in Israel. When he returns to his country, he does not need to continue the request for rain, but it is good to do so in Shome’a Tefillah (Yalkut Yosef 117:17).
In all these laws, it seems that if one erred and practiced according to one of the outlined opinions (and did not make his request in Shome’a Tefillah), he need not repeat his prayer. Concerning a chazan, even if he requests rain in Shome’a Tefillah in his silent Amidah, he prays the repetition according to the custom of the place in which he resides, because it is a prayer on behalf of the congregation (Ishei Yisrael 23:39).
. If he mistakenly requested rain in Birkat HaShanim in a place that needs rain after Pesach, according to the Shulchan Aruch 117:2 he must repeat his prayer with a stipulation that if the halachah follows the Rosh’s reasoning, then it would be permitted for the residents of that place to ask for rain in Birkat HaShanim and, indeed, this prayer is a voluntary prayer (tefillat nedavah). According to the Rama, b’dieved he is not obligated to repeat the Amidah. If he realized in the middle of his prayer that he requested rain in Birkat HaShanim at the wrong time, the Yabia Omer, part 2, Orach Chaim 9:17, based on the Shulchan Aruch, writes that he must immediately go back and correct his mistake. However, according to the Mishnah Berurah 10, since b’dieved he fulfills his obligation even after saying it, he concludes his prayer and if he wishes to repeat his prayer, he prays another Amidah as a voluntary prayer. The Kaf HaChaim 25 writes that he finishes his prayer and afterwards recites another voluntary Amidah.