Tosefta על נדרים 8:3

What is considered the second rain? Once the [normal] time for rain has arrived, the words of Rabbi Meir. And the Sages say, once the [second] rain has fallen. Rabbi Yosei says, everything that depends on rainfall, once the [normal] time for rain has arrived; and for anything that does not depend on rainfall, once the second rain has fallen. Rabban Shimon ben Gamaliel says, [as to] rains than fell for seven days, one after another, without stopping, they are [considered to be] of sufficient quantity for second rain. How much rain must fall to be [considered to be] of sufficient quantity for second rain? [Enough] to fill a vessel of three handbreadths, the words of Rabbi Meir. Rabbi Yehuda says, one [handbreadth] for the first, two for the second, and three for the "rains" (alt., "third rain"; see Lieberman). Said Rabbi Shimon ben Elazar, there is no rain as to which each and every handbreadth from above is not countered by two handbreadths from [below] the earth, and thus it says (Ps. 42:8), "Depth calls to depth [at the roar of Your cataracts]." Why is it called r'viah? Because it fertilizes (rova'at) the earth.