Mesorat Hashas sur Berakhot 9:2

How much rain must fall for one to be obligated to recite the blessing [over rainfall]? Rabbi Yosei in the name of Rabbi Yehuda and Rabbi Yona in the name of Rabbi Shmuel said: For the first rain of the season, enough to permeate [the earth], and at the end of the season,⁴¹When small amounts of rain had fallen previously. any amount. Rabbi Ḥiyya said in the name of Rabbi Yoḥanan: For the first rain of the season, enough to permeate [the earth], and at the end of the season, not until the surface of the roof tiles are rinsed off. Rabbi Yanai ben Rabbi Yishmael said in the name of Rabbi Shimon ben Lakish: For the first rain of the season, enough to permeate [the earth], and at the end of the season, enough that the sealant on top of the wine jug becomes soaked. But surely even a great deal of rain could fall and the sealant would not become soaked?⁴²Because it is waterproof. It is, rather, until it appears that it is soaked.⁴³Enough water collects on it to give it the appearance of being soaked.
What blessing should one recite [over the rain]? It is as we learned: For rain and other good tidings, one recites: ‘Blessed [is God]…Who is good and beneficent.’ Rabbi Berekhya in the name of Rabbi Levi derives this from here: “Cold water on a weary soul, so is good news from a distant land” (Proverbs 25:25) – just as for good news one recites: ‘[Blessed is God…] Who is good and beneficent,’⁴⁴Brachot 54a. for cold water [i.e., rain], too, one recites: ‘[Blessed is God…] Who is good and beneficent.’ Rabbi Yehuda said: This is the blessing that my father Yeḥezkel would recite [over rain]: May Your name be blessed, exalted, and magnified for each and every drop that you cause to fall for us, which you keep separate from one another.⁴⁵Translation follows the unanimously recommended textual emendation שאתה ממניען אלו מאלו.
Rabbi Yehuda ben Rabbi Shimon said: He rains them [the raindrops] down by measure, as it is stated: “For He deducts [yegara] drops of water” (Job 36:27), just as it says: “It shall be deducted [venigra] from the valuation” (Leviticus 27:18).⁴⁶All deduction is by measure.
Rabbi Yosei bar Yaakov once went up to visit Rabbi Yehuda of Migdal. He heard his voice reciting this blessing: ‘Thousands upon thousands and myriads upon myriads of blessings and thanks we would require to thank Your name for each and every drop that You cause to rain down for us, for You bestow good rewards [even] to sinners.’ He said to him: ‘That is the blessing that Rabbi Simon would recite.’ Rabbi Zeira said: ‘There [in Babylonia] they say: When low prices prevail in the world, when bounty comes to the world, and when a river inundates the countryside, one recites: Blessed [is God]…Who is good and beneficent.’

“Like cold water on a weary soul is good tidings from a distant land” (Proverbs 25:25) – we learned there in a mishna: Upon rainfall and upon hearing good tidings, one recites the blessing: ‘Blessed [are You]…Who is good and Who bestows good.’⁴Berakhot 54a. Why did they see fit to juxtapose good tidings to rainfall? Rabbi Berekhya said in the name of Rabbi Levi: It is based on: “Like cold water on a weary soul is good tidings from a distant land” – just as over “good tidings” [one recites:] ‘Blessed [are You]…Who is good and bestows good,’ so, too, over “cold water” [one recites:] ‘Blessed [are You]…Who is good and bestows good.’
Another interpretation, “Like cold water on a weary soul,” so is “good tidings from a distant land” – this refers to Abraham, who, while he was still at Mount Moriah, received the tidings that the [future] spouse of his son was born, as it is stated: “Behold, Milka, she too has borne…”

A man is in duty bound, etc. What means : A man is in duty bound to utter a benediction for the bad even as he utters one for the good? Are we to suppose that as for the good he says the benediction "...Who art good and dispensest good," he is to make the same benediction for the bad? For lo, our Mishnah teaches : For good tidings he says "Who art good and dispensest good" and For bad tidings he says "Blessed be the true Judge"! Raba said : It is only necessary that he should receive [the bad] with gladness.