The weight of clouds can reach quite astonishing proportions. For example, a cumulonimbus cloud, commonly known as the thunder cloud, can contain up to 300,000 tons of water.
The fact that a mass of 300,000 tons of water can remain aloft is truly amazing. Attention is drawn to the weight of clouds in other verses of the Qur'an:
It is He Who sends out the winds, bringing advance news of His mercy, so that when they have lifted up the heavy clouds, We dispatch them to a dead land and send down water to it, by means of which We bring forth all kinds of fruit... (Qur'an, 7:57)It is He Who shows you the lightning, striking fear and bringing hope; it is He Who heaps up the heavy clouds. (Qur'an, 13:12)
At the time when the Qur'an was revealed, of course, it was quite impossible to have any information about the weight of clouds. This information, revealed in the Qur'an, but discovered only recently, is yet another proof that the Qur'an is the Word of Allah.
If I were in the time of prophet Muhammad (PBUH) and I was smart I would not use the word heavy for something that is floating on the air, how did he know that if it was not a true divine revelation.