Workbook Pages

Key Ideas

• A cloud is liquid water: billions of droplets too small to fall. Fog is a cloud sitting on the ground.
• There is invisible water vapor in the air all the time. A cloud forms when rising air cools and that water collects into droplets.
• Every cloud marks a place where air is rising. The gaps between puffy clouds are where air is sinking.
• Rain happens when droplets bump and join until they are too heavy for the rising air to hold up.
• Weather arrives rather than appears. Most of it comes from fronts, where one air mass shoves another upward.

• Water vapor: Water as an invisible gas mixed into the air. It is there even on a clear day, and you cannot see it.
• Condensation: Water vapor turning into liquid droplets. It happens when air cools, and it is what makes a cloud.
• Cumulus: Puffy heaped clouds with flat bottoms and blue sky between them. They mark columns of rising air.
• Stratus and cirrus: Stratus: flat gray sheets covering the sky. Cirrus: thin wispy streaks very high up, made of ice.
• Air mass: A large region of air that has taken on the temperature and moisture of wherever it has been sitting, and then travels.
• Front: The boundary where two air masses meet. The colder air wedges underneath and shoves the warm air up, making cloud and rain.

1. Fog often shows up early in the morning and burns off by mid-morning. Using what you know about warm air and cold air, explain both halves of that.
2. Mountains often have clouds sitting on one side and clear sky on the other. Given that clouds mark rising air, what do you think the mountain is doing to the wind?
3. A forecast says forty percent chance of rain. What do you think that number actually means, and why would a forecaster give a number instead of a yes or no?

• A clear jar with a lid, warm water, a plate of ice cubes, a match, and an adult to handle the hot water and the match
• A notebook, two minutes a day, and the same time each day