Weather Basics: Why It Rains, Why It's Windy

Weather can feel random, but it comes down to two simple ingredients: water moving around, and air moving around. Once you see those two, the sky makes a lot more sense.

Start with rain. The sun heats water in oceans, lakes, and puddles, and some of that water turns into an invisible gas called water vapor that floats up into the air. This is the same thing that happens when a wet sidewalk dries in the sun – the water didn't vanish, it went up. Higher up, the air is colder, and cold air can't hold as much water vapor. So the vapor cools and gathers back into tiny droplets. Billions of those droplets together make a cloud.

When those droplets bump into each other and grow heavy enough, they fall as rain. You've seen a small version of this on a cold glass of water on a hot day – water beads form on the outside because warm, damp air touched the cold glass and the vapor turned back into liquid. A cloud is much the same thing, on a huge scale.

Now wind. Wind is simply air moving from one place to another. The sun heats the ground unevenly – a sunny field warms faster than a shady forest or a cool lake. Warm air is lighter and rises, and cooler air slides in underneath to take its place. That moving air is wind. Often, the bigger the temperature difference between two areas, the harder the wind blows. A breeze off the ocean on a hot afternoon is this exact thing: hot land air rises, and cooler sea air moves in.

Put it together and you can read the sky a little. Tall, dark, piled-up clouds mean lots of water vapor has risen fast and cooled – often a sign rain or a storm is coming. Thin, high wisps usually mean calmer weather. You won't replace a meteorologist, but you'll understand what they're talking about.