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What happens to a puddle on a sunny day? • 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? 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.