Why does it rain?

Rain is one of the most ordinary and most important parts of weather. It waters crops, fills rivers, and keeps the natural world alive. Almost everyone has watched rain fall and listened to it tap on a window. But where does all that water come from, and why does it suddenly fall from the sky? The story of rain begins high above us, inside the clouds.

A cloud may look soft and solid, but it is actually made of an enormous number of extremely tiny water droplets, far too small and light to fall. These droplets are so minute that they simply float, held up by the air, which is why a cloud can hang in the sky. For rain to happen, these tiny floating droplets need to become much bigger and much heavier. The question is how something so small grows large enough to fall.

Inside a cloud, the air is far from still. Currents push the droplets up and down, and as they move around, they bump into one another. When droplets collide, they often merge together, joining into a single larger droplet. This process is called coalescence. A bigger droplet then falls a little faster through the cloud, sweeping up even more small droplets in its path, so it grows larger and larger. Step by step, collision by collision, a droplet builds up into a proper raindrop.

A droplet can only stay up in the cloud while it is light enough for the rising air to support it. As a drop keeps growing through collisions, it eventually becomes too heavy for the air currents to hold. At that point, gravity wins, and the drop falls out of the cloud toward the ground. That is rain. A single raindrop is the result of a great many tiny cloud droplets merging together. If the air is cold enough, the falling water may instead arrive as snow or other forms of precipitation.

This article was written using information from Wikipedia.