Sometimes we see clouds in the sky and other times it’s clear with nothing except blue skies for miles. In a recent MinuteEarth (previously) episode, they explain how clouds are formed and answer the question: why are there clouds?
…So warm humid air is even more buoyant than warm dry air. As the invisible balloon goes up, the falling pressure outside allows it to keep ballooning, which spreads out its internal heat and lowers its temperature. Eventually, the air at the top cools enough for the water vapor there to condense into droplets, which from afar looks like a thin wisp of cloud. And as the rest of the balloon rises, water vapor continues to cool and condense at the same altitude, creating a flat-bottomed cloud that appears to grow upward out of nothing.