1. Unit 3
For unit objectives see: Scope & Sequence

2. Clouds

3. Review How do clouds form?
- Clouds form when the moisture in the air rises (evaporation). As large amounts of air rises it expands and gets colder. Colder air cannot hold as much water as warmer air. As the temperature and air pressure continue to drop, tiny water droplets group together into clumps called cloud droplets (condensation). At this point, the air becomes a visible cloud. If the cloud keeps going up, the cloud droplets will clump together and form water droplets. These water droplets are too heavy to float in the air and they fall from the sky as either rain or snow (precipitation).
What are clouds?
- Clouds are water. Either small liquid water drops or tiny pieces of ice.

5. Did you know clouds have names?
Cirrus
Cumulus
Cumulonimbus
Stratus
These are the basic cloud types (names),
but there are many more.

6. Cirrus Clouds
Cirrus clouds are ice clouds. They can look like delicate white feathers or streamers. They are always more than three miles up where the temperature is below freezing, even in summer. Wind currents twist and spread the ice crystals into wispy strands.

7. Cumulus Clouds
Cumulus clouds are the fluffy, white cotton ball or cauliflower-looking clouds with sharp outlines. They are "fair weather clouds" and they are fun to watch as they grow and change in shape and size. Cumulus clouds make beautiful sunsets.

8. Cumulonimbus Clouds
Cumulonimbus clouds are a sure sign of bad weather to come--at least bad if you don't like rain and hail! These clouds grow on hot days when warm, wet air rises very high into the sky. Up and down winds within the cloud may push water droplets up to very cold parts of the atmosphere, where they freeze. When the ice drops come back down, they get another coating of water and are pushed back up to freeze again. Finally, they get too heavy to stay in the cloud and fall to the Earth as hail stones.

9. Stratus Clouds
Stratus clouds often look like thin, white sheets covering the whole sky. Since they are so thin, they seldom produce much rain or snow. Sometimes, in the mountains or hills, these clouds appear to be fog.