Why Afternoon Clouds Have Flat Bottoms

On a sunny morning, the sky may look clear. By afternoon, small white clouds can appear above a field. These are cumulus clouds, often shaped like heaps of cotton. Their tops look rounded, but their bottoms may look surprisingly flat. Why? To solve that puzzle, follow the air from the ground upward.

Sunlight warms the ground. The ground then warms the air touching it. Some patches of land get warmer than nearby patches. Above a warm patch, a pocket of air can become warmer and less dense than the air around it. This lighter pocket begins to rise, carrying moisture upward.

That moisture includes water vapor, which is water in its gas form. Water vapor is invisible. It gets into the air when liquid water evaporates from places such as puddles and damp soil. Plants also release water vapor through their leaves. Even when a blue sky looks empty, the air can contain this unseen water.

As the pocket rises, it expands, or spreads out, because the air pressure around it is lower higher up. Expanding air cools. If it cools enough, some water vapor condenses: it changes into tiny liquid droplets on particles such as dust. Many droplets together scatter sunlight, making a white cloud visible. The cloud is not a heap of water vapor.

Nearby pockets of rising air often cool enough to form droplets at about the same height. That shared starting level gives the clouds their fairly flat bottoms. Above that level, rising air can keep building rounded tops. A small cumulus cloud may form without producing any rain.

A diagram can help solve the flat-bottom puzzle. Its heading is “From Warm Ground to White Cloud.” Arrows lead upward from sun-warmed soil to a cloud. A line marks where droplets begin forming. The caption says, “The cloud becomes visible where rising air has cooled enough.” The line explains the bottom; the arrows explain how air gets there.