During cloud formation water can become supercooled, that is, it can get below freezing point but remain not frozen. This stage is so unstable that we can add some catalyst molecules to trigger rain. Historically humans used silver iodide, frozen carbon dioxide and some salts to trigger rain. This is known as cloud seeding.
However nature uses different material to seed the clouds. Nature uses both sea salt and land dust to trigger rain: Wind at the surface of the seas causes waves to roll. Those waves release aerosols into the air carrying the sea salt. The wind carries both the sea salt and the land dust to the clouds. The sea salt and land dust cause the supercooled water to coalesce and trigger precipitation.
[Quran 15:22] And We send the seeding winds, then cause the rain to descend from the sky, therewith providing you with water, though you are not the guardians of its stores.
How could an illiterate man who lived 1400 years ago have known that wind has a seeding role in clouds?