Earthsky FAQs
Private: Why are some clouds flat on the bottom?
07-24-2008 - Earth
Certain kinds of fair-weather clouds – cumulus clouds – fit that description. These clouds form when warm air from Earth’s surface rises and cools. Even in the driest climates, there’s always some water vapor in the air. As you go up in the atmosphere, and the air cools, this vapor condenses to form water droplets. It happens because cool air can’t hold as much moisture as warm air. And the height at which it happens depends on the temperature and moisture content of the air.
So, it’s no optical illusion – but a fact that the bottoms of puffy cumulus clouds all lie at about the same height above Earth’s surface. That’s because – as soon as air gets high enough above Earth’s surface to cool to just the right temperature – water vapor in the atmosphere transforms to water droplets – and a cloud is born.
Cumulus clouds may float higher and higher throughout the day, as sunlight heats Earth’s surface – and warm air has to rise higher before it’s cooled enough to make a cloud. You can see the upward motion of air from Earth’s surface in the puffy, cotton ball shapes of cumulus clouds.