Why do droplets of water form defined clouds rather than dispersing evenly into the atmosphere?

Answer 1

Droplets of water are not alone, they get mixed with dust and smoke particles and become dense to form clouds at cooler heights.

Answer 2

The way puffy cumulus clouds form - with their well-defined borders, edges, tops and bottoms - depends on very localized differences in atmospheric conditions that reflect differences in how the ground below absorbs heat, said Geoff Cornish, a Penn State meteorologist.

Different surfaces absorb and emit the sun's rays in different ways, Mr. Cornish said, ocean versus land, or on a smaller scale fields versus forest. The surfaces take on different temperatures and impart them to the air above.

"The atmosphere is a complicated fluid," he said, "and within this fluid localized areas of air are forced upward." Then, as air rises and cools, the humidity rises because warm air has a greater capacity for water vapor than cool air.

Eventually the humidity reaches 100 percent, the saturation point, and the moisture condenses onto cloud condensation nuclei, specks of dust in the atmosphere.

Clusters of millions of cloud droplets form in the updrafts.

Over a larger area, Mr. Cornish continued, air cannot move upward without compensating subsiding air. This results in the clearly defined edges seen in cumulus clouds. The bottoms of clouds are where the parcel of air becomes saturated, and the top is where the upward impulse dies out, he said.

These principles are at work no matter how large a cloud is, he said. In a hurricane or very large winter storm, clouds may be hundreds of miles across, with much larger areas of lift and surrounding subsidence..