All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
These mountains mentioned in the verse are actual mountains in their shape but they are not from rocks or stones; rather, they are cumulus clouds that grow vertically carrying hailstones. They have the shape of huge mountains. Shaykh Mahmood Shukri Al-Aloosi said in his book, Ma Dalla 'alayhi al-Quran mimma Yu'adhdhid al-Hay’ah al-Jadeedah al-Qaweemah bil-Burhaan: “Tafseer scholars differed in opinion about what is meant by these mountains. Mujaahid, Al-Kalbi and Tafseer scholars are of the view that what is meant by 'heaven' is a canopy, and what is meant by 'mountains' is real mountains; they said: Allaah created in the sky mountains of hailstones as He created mountains of stones on earth, and there is nothing that rationally contradicts this, so it is permissible to understand [or interpret] the verse according to its apparent meaning.”
Shaykh Az-Zindaani said: “Let us leave the interpretation of this verse to the Head of the Department of Astronomy, Faculty of Science, Cairo University, Dr. Jamaal Al-Deen Al-Findi, who says: 'I tell my students that we should mention this verse among scientific reference lists as the first thing we knew about the facts relating to those clouds. We must make this clear to Western scientists who have studied and researched clouds so that they realize that the Quran took the lead in science. A characteristic of cumulus clouds is that they may extend vertically to a height of fifteen kilometers or more, and thus they appear to those who look at them from a distance as if they are huge mountains. The ability to grow vertically provides the possibility of the formation of cumulus clouds across the layers of the atmosphere whose temperatures vary greatly, and as a result, vertical swirls are formed and hailstones are formed as well; so the cumulus clouds are the only ones that produce hailstones and this fact in itself explains the verse in which Allaah, The Almighty, says (what means): {… And He sends down from the sky, mountains [of clouds] within which is hail….} [Quran 24:43] When the scientists used radar to illustrate the stages of the formation of cumulus clouds in the aftermath of World War II, they found that clouds begin in the form of several cells or units of clouds raised by air currents; this is followed by the merging of every two cells or more, depending on the circumstances, to form the rainy cumulus clouds. Although man did not discover this amazing scientific fact until recently, we find that the Quran documents this fact in a simple way, as Allaah says (what means): {Do you not see that Allaah drives clouds? Then He brings them together, then He makes them into a mass.} [Quran 24:4] The first part of the verse tells us the stages of the formation of cumulus clouds and then specifies that they grow vertically until they become like mountains; at this point they produce hail, unlike other kinds of clouds. The hail does not necessarily fall from the cloud as soon as it is formed; as the current of ascending air may prevent it from falling in a particular place, but when this current becomes weak, the hail descends in the form of a relentless cumulus, as if the cloud bursts; this explains to us the saying of Allaah (which means): {…and He strikes with it whom He wills and averts it from whom He wills…} [Quran 24:43]"
Professor Muhammad Qutb said in his book (They could not produce the like of it): “Cumulus clouds do not appear in their real shape to people looking at them from below, i.e. from the surface of the earth, as what is apparent is only the lower base of the clouds, but they extend vertically in the sky. However, when you get to the top [i.e. above the clouds], in a plane for example, you will see the accumulation of these clouds, one on top of the other, so you would see them in their real form, and you would see that they are layers, rather than a single layer as they appear to people looking at them from the surface of the earth. They are not all of the same height, but the height of their layers differ according to the accumulation of water vapor in each layer, and some of them appear like mountains with peaks of different heights, hanging from the sky!”
Researcher, Ashraf Amaasha, said in his Master thesis (The Universe and the Scientific Perspective in the Quran and Other Heavenly Religions): “Perhaps the first book which documented the fact that the rainy clouds are driven by wind is the Quran. The perception among some ancient civilizations is that rain comes from water stored in the sky when the gods open the doors and windows! Let us study the saying of Allaah (which means): {It is Allaah, who sends the winds, and they stir the clouds and spread them in the sky} [Quran 30:48], and the saying of Allaah (which means): {And it is He who sends the winds as good tidings before His mercy [i.e. rainfall] until, when they have carried heavy rain clouds, We drive them to a dead land and We send down rain therein and bring forth thereby [some] of all the fruits. Thus will We bring forth the dead; perhaps you may be reminded.} [Quran 7:57] Here, it should be noted that at the time when the Quran was revealed, no one knew that air carries great amounts of water in the form of water vapor, and that it is this water vapor which forms the clouds and gives rain when air currents go up and become cold under the influence of proliferation and the reduction of the air pressure because of higher altitude. So the water is the basis for the formation of the clouds due to the existence of many factors that meet and interact together forming the clouds....The clouds either grow vertically like mountains, in which case they are called cumulus, or grow horizontally, stretching like a blanket and are then called stratus clouds. The Quran differentiates between the two types, as it calls the first type cumulus and the second type stratus. Indicating stratus clouds, Allaah says (what means): {It is Allaah, who sends the winds, and they stir the clouds and spread them in the sky however He wills.} [Quran 30:48]. The Quran also states the advantages of the former kind of clouds, i.e. the cumulus clouds, clarifying that they are the only clouds that forms hail as well as thunder and lightning storms; Allaah says (what means) {And He sends down from the sky, mountains [of clouds] within which is hail.} [Quran 24:43]"
Allaah Knows best.