Explain humidity as the amount of water vapor in air and describe how clouds form when moist air cools to its dew point.
Humidity measures how much water vapor is mixed into the air around you. Warm air can hold much more water vapor than cold air, which is why a hot, sticky summer afternoon often feels heavy and a cold winter day rarely does. When air is holding close to the maximum water vapor it can hold at that temperature, the humidity is high.
Clouds form when moist air cools down to its dew point, the temperature at which the air can no longer hold all its water vapor as gas, so some of it condenses into tiny liquid droplets. Those droplets need something to cling to, usually tiny bits of dust, salt, or pollen floating in the air, called condensation nuclei. Billions of these droplets clumped together, floating, is what you see as a cloud.
Mastery looks like: They can explain, in their own words, why moist air cooling to its dew point produces visible droplets, and can point to a real example of condensation.
Common stumbles: Confusing humidity (water vapor in air) with plain temperature; thinking clouds are made of steam rather than tiny liquid droplets.