1) In Eretz Yisrael we start asking for rain by saying “Vesein Tal Umatar Livrachah” (and give dew and rain for blessing) in the Amidah from the seventh of Cheshvan, because Chazal estimated that by this date even the last members of Klal Yisrael who came up to Yerushalayim for Sukkos would have already crossed the border to return to their homeland, and the rain wold not disturb them on their way. (Shulchan Aruch O”Ch 117)

2) One who did not say “Vesein Tal Umatar Livrachah” must return and repeat it according to the list of rules which will soon be delineated. The reason for this is because this is a request and in prayer one must request each detail separately. (As opposed to “Mashiv haruach umorid hagashem” which is a praise and therefore if he has already praised Hashem by saying “Morid Hatal” he does not have to repeat it). (Mishne Berurah ibid)