We have pointed out that some of Chazal’s descriptions of “the” rakia refer to the atmosphere, and some refer to the celestial spheres. Sometimes it is not easy to determine which are which, and rishonim differ. However, there definitely are scriptural and rabbinical descriptions that are indisputably about the cloud region of the sky. And among these are some that clearly use terms associated to texture. Let us see, when they speak of the sky being like one or another texture, whether the terminology is meant in the unsophisticated sense Rabbi Slifkin imputes to the same kind of terminology when it is applied to “the” rakia of the sun, moon and stars.
The Chumash Describes the Rakia’s Texture
Here is what pesukim say about the sky’s texture:
וְשָׁבַרְתִּי אֶת-גְּאוֹן עֻזְּכֶם וְנָתַתִּי אֶת-שְׁמֵיכֶם כַּבַּרְזֶל וְאֶת-אַרְצְכֶם כַּנְּחֻשָׁת
And I shall shatter your proud strength, and I shall put your heavens like iron, and your land like copper (Vayikra 26:19).
Now, even if, ala Rabbi Slifkin, one were to take the reference to “iron” as an attribution of hardness—which we shall show is specifically here not the case—it would only show that this hardness is precisely not the normal, default or original state of the rakia. “Iron-like” is only a consistency with which Hashem endows the rakia at times that the people neglect the Torah. Otherwise the rakia is not “iron-like”—not hard; or not, at least, as hard as iron!
The default status of the rakia, also called shechakim is, as the word connotes, an area of the sky that looks like “pulverized” water and fire—one might say steamy or misty.
As for Chazal, The Sifri comments on the verse:
ועצר את השמים"--שיהו עננים טעונים גשמים ואין מורידים גשמים אפילו טיפה אחת. ומנין אפילו טללים ורוחות? ת"ל: “ונתתי את שמיכם כברזל” (ויקרא כ"ו). (אינו אלא) בית השלחין יהיה עושה פירות ת"ל "ואת ארצכם כנחושה" (סיפרי פרשת עקב פיסקא מג). ס
[The Torah says: “If you will not keep the commandments] Hashem will stop up the heavens (shamayim)…” This means that the clouds will be bearing the rainwater, but not let them fall as rain—not even one drop. And not even dew or winds, for it says, “And I will make your heavens (shamayim) like iron. Nor will your irrigated areas produce fruit, for it says, “… [and I will make] your land like copper.”
In this passage, Chazal take “shamayim” to refer to the “lower rakia,” the one of the clouds.—and its rainwaters normally have no trouble flowing through the rakia to earth.
So in reality, in what aspect do we see that the pesukim and Chazal saw the shamayim, or clouds, when “turned into” iron, to be like iron?—in the aspect that they will not exude water; not in the aspect of their material consistency or texture.
The concept is repeated with more detail by Chazal in their comments on the frightening tochacha passage (Vayikra 26:19) where Hashem tells us that if we refuse to repent,
וְשָׁבַרְתִּי אֶת- גְּאוֹן עֻזְּכֶם וְנָתַתִּי אֶת-שְׁמֵיכֶם כַּבַּרְזֶל וְאֶת-אַרְצְכֶם כַּנְּחֻשָׁה
I will shatter the glory of your strength, and make your shamayim like iron, and your land like copper.
(ג) ... ונתתי את שמיכם כברזל ואת ארצכם כנחושה. שלא יהא השמים מזיעים כדרך שאין הברזל מזיע. והארץ תהא ממזיע כדרך שהנחושת מזיע. והיא מאבדת פירותיה.
“…I will place your shamayim like iron, and your land like copper.”—Meaning, the shamayim [the region of the sky from which rain forms, or the clouds] will not sweat, just as iron does not sweat. And the land will sweat, just as copper sweats, and ruin its produce.
Again, the aspect to which the iron-like or copper-like refers is not texture. Here, again, it refers to the aspect, or lack thereof, of exuding liquid. The point being, the result will be absence of rain. (The commonality of result in a metaphor does not compel the commonality of cause. If one says that another’s brain is hard as a rock, the accuser’s point is that his message is not being processed by the accused’s thinking; it does not mean to accurately describe the texture of the accused’s brain, or even the means by which thoughts are processed or blocked.)
When metaphors and similes are made, one must investigate the aspect in which the equivalency is meant. When Yaakov calls Yehuda a “young lion (גור אריה), or Binyamin a “wolf,” one realizes he did not think his sons were animals, and one strives to understand what aspect of a lion or wolf he had in mind.—His facial appearance? His strength? His method of attack? His royalty? His cunning? His ferociousness? So too, when the rakia is referred to as “like brass,” “strong,” “like a mirror,” “a plate,” “a tent,” etc., one must ascertain in what aspects the equivalence is meant, and not attribute equivalence in other aspects.
This is of course elementary, and should not be forgotten when dealing with pesukim and Chazal, despite one’s agenda. And the place to go to see what aspect Chazal or pesukim intended by their similes and metaphors, is Chazal and pesukim themselves, guided by the rishonim—not Christian evangelists.
 See Addendum for an interesting suggestion that the word “sky” is related to the Hebrew “shechakim.”
 The Rambam has already pointed out that the Torah uses the words shamayim and rakia interchangeably regarding the two rakias. Here we have a passage in which Chazal clearly identify shamayim with the clouds.
Interesting Suggestions about the Etymology of the Word “Sky” From the Hebrew “Shechakim” (from http://www.edenics.net/english-word-origins.aspx?word=OBSCURE)
It has been suggested that the English word “sky” ultimately derives from שחק: Latin obscurus is from ob (toward) + the Indo-European “root” (s)keu (to cover, conceal).
שחק SHaK[H]aQ is a cloud (Psalms 77:18); the plural שחקים SHiK[H]aKiM means clouds, heaven or "skies" (Deuteronomy 33:26) .
סכך SaKHaKH is to screen or cover over. שחר SHaK[H]oaR is dark or "black" (Leviticus13:31); The link to SKY is OBSCURED by the fact that ש-ח-ק Shin-Het-Koof originally meant cloud of dust
The connection is clearer when considering that SKY, from Germanic skeu-jam (cloud), is a cognate of OBSCURE. Sky in Danish and Norwegian. Similar in Icelandic, means “cloud.”)