Have you ever looked up on a beautiful summer afternoon and noticed the sky filled with fluffy, cotton-like clouds? They're called fair-weather cumulus clouds, and they form because of one powerful energy source: the sun!
The Sun begins warming the Earth's surface each morning. But not everything heats up at the same rate. Asphalt, sidewalks, rooftops, and parking lots absorb heat much faster than grassy fields or lakes. As the ground warms, it also heats the air directly above it.
That warm air becomes less dense than the cooler air around it, causing it to rise in invisible pockets known as thermals. Thermals are like bubbles of warm air floating upward through the atmosphere. They eventually move into areas where the air pressure is lower. Because of this lower pressure, the air expands and cools. Eventually, the rising air cools enough to reach its dew point—the temperature at which the air can no longer hold all of its water vapor. At that moment, the invisible water vapor begins condensing into millions of tiny water droplets.
Those tiny droplets gather together, and suddenly a bright white cloud appears overhead. That's the birth of a fair-weather cumulus cloud!
If the atmosphere is relatively stable and there's limited moisture, these clouds stay small and harmless. However, if there's plenty of moisture and the atmosphere is unstable, those same little cumulus clouds can continue growing into towering cumulus clouds—and eventually even thunderstorms.
These innocent-looking clouds can actually tell meteorologists quite a bit about what's happening in the atmosphere!
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