Water droplets form through a process called condensation. This happens when water vapor, which exists as an invisible gas in the air, cools down and changes state from gas to liquid. When warm, moist air comes into contact with a cold surface, the water vapor loses energy, slows down, and transforms into liquid water droplets.
In the atmosphere, clouds form when water vapor rises and cools. But water vapor doesn't simply turn into droplets on its own. It needs something to condense onto. These are called condensation nuclei - tiny particles like dust, salt, or smoke that are suspended in the air. Water vapor molecules attach to these particles, forming tiny cloud droplets. Without these nuclei, cloud formation would be much more difficult.
Once tiny cloud droplets form, they continue to grow through two main processes. First, more water vapor condenses onto the existing droplets, making them gradually larger. Second, as these droplets move around in the cloud, they collide with each other. When droplets collide, they often merge together in a process called coalescence. This creates fewer but larger droplets. Through repeated collisions and continued condensation, some droplets eventually grow large enough that gravity's pull on them becomes significant.
Eventually, some water droplets in the cloud grow large enough that gravity's pull on them becomes stronger than the upward air currents that keep them suspended. When a droplet's weight overcomes these updrafts and air resistance, it begins to fall toward Earth as rain. As the droplet falls, it may continue to grow by collecting more water. This process completes the transformation from invisible water vapor to visible raindrops. When these droplets reach the ground, they've completed their journey from gas to liquid, forming the water droplets we see as rain.
To summarize what we've learned about water droplet formation: First, water droplets form through condensation when water vapor in the air cools and changes from gas to liquid state. In clouds, this process begins when water vapor condenses onto tiny particles called condensation nuclei, such as dust, salt, or smoke. These tiny cloud droplets then grow larger through continued condensation and by colliding and merging with other droplets. Finally, when the droplets become heavy enough, gravity pulls them down as rain. This entire process is a fundamental part of Earth's water cycle, continuously moving water between the atmosphere and the Earth's surface.