1. Cloud Types and Formation, Water Cycle, Precipitation
Chapter 7
Lesson 2

2. Lesson 2 a How does the water cycle affect weather? cloud forming
warm air
warm air
cloud forming
cloud forming
warm air
cool air
Discuss the Main Idea
What three forms does water have?
Liquid, solid ice, gas vapor
What causes water to change form?
Changes in temperature
Develop Vocabulary
Evaporation: Scientific vs. Common Use Tell students that the word evaporation means “to change from a liquid to a gas.” Sometimes to evaporate in common language means “to disappear,” as in, “Our fears evaporated when the lights came on.” However, it is important to realize that, scientifically, when water evaporates, it does not disappear; it only changes form.
Condensation: Word Origin The word condensation is a form of the word condense. Condense comes from the Latin prefix com- and the root densare, which means “to make more compact.” Liquid water molecules are closer together than water vapor molecules, so liquid water is more compact than water vapor.
Humidity: Tell students that they have probably experienced high humidity is usually high after a rainstorm if the temperature remains hot.
Use the Visuals
Have students look at the visuals of cloud formation on page Ask:
In which direction do warm air and cold air move? Warm air moves upward, cold air moves downward.
How does a cloud form? When warm, moist air rises and cools, small water droplets form a cloud.
Quick Check (Critical Thinking)
How are humidity and relative humidity alike? How are they different?
They are alike because both indicate the amount of water vapor in the air. However, they also are different. Humidity is absolute, based on the actual percent of water vapor in the air. Relative humidity depends on how much water vapor the air could hold at that particular temperature before it becomes saturated at its dew point. Actual humidity is measured in grams of water vapor per unit volume, and relative humidity is measured in percent.
Lesson 2 a

3. - Water in the Atmosphere
How Clouds Form
Clouds form when warm, moist air rises and cools. Water vapor condenses on tiny particles to form liquid water or ice crystals.

4. - Water in the Atmosphere
Types of Clouds
Scientists classify clouds into three main types based on their shape: cirrus, cumulus, and stratus. Clouds are further classified by their altitude.

5. Lesson 2 b What are the types of clouds? high clouds middle clouds
13 km
cirrocumulus
cirrostratus
high clouds
cirrus
cumulonimbus
5 km
altocumulus
middle clouds
altostratus
Discuss the Main Idea
What are the three main classifications of clouds?
Stratus, cumulus, cirrus
Develop Vocabulary
Have students make index cards with pictures of clouds and their characteristics on one side and their names on the back. Have them practice identifying types of clouds according to their shapes, heights, and characteristic.
Stratus: Word Origin Tell students that the word stratus is Latin for “spread” or “bed.” Scientists named stratus clouds because their structure looks like a blanket or layer.
Cumulus: Word Origin Tell students that the word cumulus is Latin for “pile.” Scientists named cumulus clouds because their structure looks like a bunch of piled-up material.
Cirrus: Word Origin Tell students that the word cirrus is Latin for “hair.” These stringy clouds were named because they resemble wispy strands of hair.
Quick Check (Critical Thinking)
Compare altocumulus and altostratus clouds.
Both are formed at the middle-level altitudes. Altocumulus are puffy. Altostratus are blanketlike layers.
stratocumulus
2 km
cumulus
low clouds
stratus
nimbostratus
0 km
Lesson 2 b

6. - Precipitation
Water Droplets
Droplets come in many sizes. A raindrop has about one million times as much water in it as a cloud droplet.

7. Lesson 2 c What are the different types of precipitation? Rain Sleet
Discuss the Main Idea
In what form does all precipitation begin?
Water droplets or ice crystals in clouds
What determines what form the precipitation will be as it falls?
The surrounding temperature
Develop Vocabulary
Precipitation: Have students make a table classifying the different forms of precipitation. Have them list the types precipitation in one column, the physical state of the precipitation in one column, the physical state of the precipitation in a second column, and the conditions that form it in the third column.
Quick Check (Critical Thinking)
Why do you think hail forms in tall clouds?
The tall clouds have different temperatures at different heights. As ice crystals in the clouds are pushed up high and them fall down again, they freeze. Then they are coated with water and freeze again.
Hail
Snow
Lesson 2 c

8. - Precipitation
How Hail Forms
Hailstones start as small pellets of ice in cumulonimbus clouds. They grow larger as they are repeatedly tossed up and down until they become so heavy they fall to the ground.

9. Asking Questions - Water in the Atmosphere Question Answer
How does the water cycle work?
Water evaporates from the surface, condenses to form clouds, and falls to Earth as rain or snow.
What is relative humidity?
The percentage of water vapor in the air compared to the maximum amount air can hold at that temperature
How do clouds form?
Water in the air condenses on tiny particles in the air to form liquid water or crystals.
Can you determine weather conditions by looking at clouds?
Yes; each type of cloud is associated with a particular type of weather.