Deuteronomy 28:23

והיו שמיך אשר על ראשך נחשת AND THY HEAVEN WHICH IS OVER THY HEAD SHALL BE COPPER — These curses (i.e. all the curses contained in this chapter) Moses expressed them as though they came from his own mouth (Megillah 31b) whilst those he spoke on Mount Sinai (Leviticus 26), he expressed them as though from the mouth of the Holy One, blessed be He, as indeed is implied by them (by the expressions used), for so it states there: “And if ye will not hearken unto Me”; “And if ye walk contrary to Me”, whilst here it says: “[And if thou wilt not obey] the voice of the Lord thy God”; “The Lord shall make cleave unto thee”; “The Lord shall smite thee”. Moses was milder in his curses, expressing them in the singular (“The Lord shall smite thee”, not “you”, the entire people), and similarly in this curse, too, he was milder, for in the former curses (those in Leviticus) it states, (Leviticus 26:19) “[I will make] your heaven as iron and your earth as copper”, i.e., that the heaven would not exude moisture, just as iron does not exude, and there would therefore be drought in the world, whilst the earth would exude moisture (be humid) just as copper sweats, and it would consequently rot the fruits. Here, however, it states, “Thy heaven shall be copper and thy earth shall be iron”, i.e. that the heavens would exude moisture: even though they might not pour with rain, at any rate there would be no ruinous drought in the world; whilst the earth would not exude moisture just as iron does not sweat, and consequently the fruits would not perish. Nevertheless this is a curse: for whether it (the earth) be as copper or whether it be as iron it will produce no fruit, and the heaven, too, will not pour with rain (give rain in abundance) (cf. Sifra, Bechukotai, Chapter 5 3 on Leviticus 26:19).

And thy heaven that is over thy head shall be brass, and the earth that is under thee shall be iron.

וְהָי֥וּ שָׁמֶ֛יךָ אֲשֶׁ֥ר עַל־רֹאשְׁךָ֖ נְחֹ֑שֶׁת וְהָאָ֥רֶץ אֲשֶׁר־תַּחְתֶּ֖יךָ בַּרְזֶֽל׃