Let’s start with the short answer and expand from there. Clouds are formed from water vapor. When water evaporates from the earth’s surface (from lakes, rivers, oceans, etc.), it rises into the air in the form of vapor. As it rises, the water vapor encounters less air pressure, so it’s able to expand. This is the first “step” in how clouds form in the sky.

The second step is when the air cools and condenses. When this happens, the water changes its state again — this time going from a vapor to a liquid. So when you see a cloud in the sky, you are actually seeing tiny water droplets that have formed together. It looks gaseous, like smoke, but it’s actually water in liquid form.

Why are some days cloudy and other not? Because there has to be something in the air for the water droplets to “cling” to, and for a cloud to take shape. This might be dust particles, smoke from fires, or salt from oceans. But there has to be some kind of medium for the droplets to connect with. The temperature also has a lot to do with this process. When the conditions are just right for this process, clouds will form in the sky. When the conditions are not right, you can enjoy a nice sunny day.

This is how clouds form, and it’s also a good illustration of the endless water cycle on our planet: Water evaporates from lakes, rivers and oceans >> it rises into the air as vapor >> it condenses back into liquid again >> it forms clouds >> it eventually comes back down as rain. And then the process starts all over again!