1400 years ago people thought that clouds are weightless. Today we know this is false. Clouds turned-out to be very heavy.
How much does a cloud weigh?Do you think clouds have any weight? How can they, if they are floating in the air like a balloon filled with helium? If you tie a helium balloon to a kitchen scale it won't register any weight, so why should a cloud?... Doing the math: 1,000,000,000 x 0.5 = 500,000,000 grams of water droplets in our cloud. That is about 500,000 kilograms or 1.1 million pounds (about 551 tons).
So the clouds turned-out to be very heavy. Average cloud above your head weighs about 100 elephants. This was known recently, however this was portrayed in the Quran 1400 years before it was discovered.
"Thikal ثِّقَالَ" means heavy. Here the clouds have weight. Today we know that clouds are not weightless as previously thought but rather are very heavy.
How could an illiterate man who lived 1400 years ago have known that clouds have weight?
The Quran (Koran, the book of Islam) contains scientific knowledge that could not have been known 1400 years ago. It ranges from basic arithmetics to the most advanced topics in astrophysics. You are invited to go through those miracles and judge for yourself.