The firmament is similar to a pool of water and above the pool is a dome. Because of the pool there is condensation of thick drops, and they descend into the salt water but do not intermingle. Rabbi Yona said: Do not wonder; the Jordan passes through the Sea of Tiberias but does not intermingle [with its water]. There is miraculous process involved in this matter.12The matter of rainfall.
When a person sifts wheat or stubble in a sieve, before they fall two or three fingerbreadths they are already intermingled, but these [raindrops] travel a distance of several years and do not intermingle with one another. Rabbi Yudan ben Rabbi Shimon says: He rains them down by measure, as it is stated: “For He deducts [yigara] drops of water” (Job 36:27), just as it says: “It will be deducted [venigra] from your valuation” (Leviticus 27:18).13All deduction is by measure.
The thickness of the earth is equivalent to the thickness of the firmament, as it is stated: “It is He who sits over the circuit [ḥug] of the earth…” (Isaiah 40:22), “in the circuit [ḥug] of the heavens He makes His way” (Job 22:14) – the words ḥug ḥug provide a verbal analogy.14Establishing a comparison between the earth and the heavens. Rav Aḥa said in the name of Rabbi Ḥanina: It is like a sheet of metal.15The firmament is not as thick as the earth; rather it is extremely thin.
Rabbi Yehoshua bar Rabbi Neḥemya said: It is approximately two or three fingerbreadths [thick]. Rabbi Shimon ben Pazi said: The upper waters contain approximately thirty kesustas16A certain measure of volume. more than the lower – “and let it divide between water and water [lamayim]” – lamed equals thirty.17The numerical value of lamayim is thirty more than the numerical value of mayim. The Rabbis say: It is half and half.