According to NASA, "A cloud is a mass of water drops or ice crystals suspended in the atmosphere. Clouds form when water condenses in the sky. The condensation lets us see the water vapor."
Analyzing NASA's definition alone, clouds are made of water vapors, which is a gas and a gas can't be seen, condenses into water or ice. In other words, we can see clouds since the water vapor has changed its phase into liquid (water) or/and solid (ice).
On the other hand, other sources will tell you clouds is a combination of the three phases of matter: solid in ice, a liquid in water, and gas in water vapor. Anyways, that does not change the fact that you can see liquids and solids.
As for air, it is only a collection of gases. And gases, as you know, is invisible to human eye.
Hope that helps.
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