system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does water vapour form distinct clouds rather than just spread out more evenly in the sky?
gpt
Sometimes it does spread out more evenly, and then you get a grey overcast day.

Clouds form when the temperature falls below something called the "dew point", or when pressure falls. Cold thin air can hold less water vapour than warm air, and so when the temperature and/or pressure falls below a certain point the invisible water vapour that's in the air and contributes to something called "humidity" starts congealing into tiny floating droplets instead. 

You get this as condensation and dew too, just that doesn't float. Take a cold surface such as a glass of water with an ice cube on it and the cold glass causes condensation to occur there. Ditto dew on grass after a cold night.

The air is filled with regions of warm and cold. On a hot day, thermals cause warm humid air to rise into the atmosphere, injecting upwards into colder air regions with less air pressure. The water vapour comes out into visible droplet form, and a cloud forms. They're poofy and clumped rather than perfectly distributed because that warm air rises unevenly, more in some places than others. 

Or a plane flies along spewing warm moist air out of its engines and you sometimes get a contrail.