A normal hour
Air gets cooler as you go up
On an ordinary day, each kilometer up is about 6.5°C cooler. Warm air near the ground can rise through that cooler air, so moisture, smoke, and heat drift away.
Lid
Lift your head above the clouds.
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What a lid is
A lid
Warmer air sitting on colder air
On a clear, still night the ground gives up its heat and chills the air resting on it. The air above stays warmer, and the colder air is too heavy to rise through it. Fog, frost, and haze then collect underneath.
A lid can hold cloud in the valleys and glens, and leave the mountain tops clear.