How clouds form

Cumulus clouds, the most common type of cloud, start to form when energy from the sun reaches water on the Earth’s surface — water in the oceans, lakes, and rivers. This solar energy breaks apart the bonds holding the liquid water molecules together. When the water molecules are no longer held together by the bonds, the liquid water turns into a gas — the water evaporates. The gas is called water vapour. The warm air filled with water vapour starts to rise… but why?

When air is heated, the molecules that make up the air jiggle and zip around faster. They spread out. When a mass of air takes up more space, it has lower density. And when a lower density gas is surrounded by a higher density gas — in this case cooler air — the lower density gas rises. Air containing water vapour, humid air, rises too.

The molecules that make up water contain two hydrogen and one oxygen atom. They are lighter than the molecules that make up air, which contains molecules of two nitrogen and two oxygen atoms. So warm, humid air rises even faster than warm, dry air. As the warmed, humid pocket of air rises, cooler air surrounds it and sinks below it. The cooler air pinches off the pocket and pushes it afloat, like an invisible hot air balloon.

In fact, the more water vapour a pocket of air collects before it lifts off, the lighter it gets. As the air pocket rises, it slowly cools down. The molecules of water vapour at the top of the pocket turn back into tiny droplets of liquid water — they condense. These water droplets suspended in the air, look like… you guessed it, a cloud! As the rest of the air pocket rises, it likewise cools and condenses.

This is why we see flat-bottomed clouds that appear to grow upward out of nothing. As the condensing process continues, the water molecules release the solar energy they absorbed when they evaporated. This energy heats the air surrounding the newly formed cloud. And because hot air rises, it pushes the whole cloud up, lifts it. We call this force uplift.

As the cloud rises, more warmed air from beneath it rises, cools, and condenses, joining the cloud. The cloud grows larger and larger… and heavier and heavier. Eventually, the weight of the cloud's droplets means the force of gravity acting upon them is greater than the force of uplift. The cloud’s water droplets fall back to the Earth’s surface...