On a sunny day, water from a puddle evaporates. Evaporation is liquid water dissolving into the air. It becomes water vapor. The water behaves like a gas, but it does not change to steam. (Have you ever seen a puddle boil?) What happens to the water vapor? What happens to a puddle on a sunny day?

Clouds

Evaporation Imagine a mass of air near the ground. Water is evaporating into it. The air mass is becoming more humid. At the same time, the air is being warmed by heat from the ground. What happens to warm air?

What will happen to the air as it rises? As you move up through the troposphere, temperature decreases. As the warm, humid air rises, it begins to cool.

Which holds more water vapor, warm air or cold? Warm air holds more water than cold, so as the air rises and cools, its relative humidity increases. When the air mass’s temperature reaches the dew point, some of the water vapor has to condense.

Condensation Just like morning dew, the water vapor needs something to condense on. Tiny droplets of water begin to form around any small piece of dust, dirt, or smoke in the air. (If the air is cold enough, an ice crystal forms instead.) The droplets are small enough to float in the air. (remember colloids?)

Condensation Water begins to condense onto to a seed of dirt, dust, etc. forming a droplet. As more water condenses onto the droplet it grows. In cold air, the water freezes to the seed, forming a snow flake. Hailstones form in a similar way, but grow larger due to high winds in thunderstorms.