24.4 Water in the Atmosphere
Reading Focus
Key Concepts
What are some ways in which water condenses in the atmosphere?- How do clouds form?
- What are the major types of clouds?
- What are the major forms of precipitation?
Vocabulary
humidity
relative humidity
dew point
cloud
fog
stratus clouds
cumulus clouds
cirrus clouds
Reading Strategy
Sequencing Copy the flowchart below. As you read, complete it to show how a cloud forms. Add more steps to the chart if necessary.
Cloud Formation
Figure 16 This antique “weather house” shows the humidity, or moisture content, of the air. If the air is humid, a hair inside the house expands and allows the man to come out of the door. When the air is dry, the hair contracts and the woman comes out.
What do fogged-up windows on a cold winter morning and a bad hair day have in common? Both result from high levels of water vapor in the air. The atmosphere contains only about 0.001 percent of the total water in the world. Yet this relatively small amount of water has a great effect on the processes that occur in the atmosphere, including cloud formation and precipitation. It can also cause your windows to fog up and your hair to become limp or frizzy.
Humidity
The amount of water in the atmosphere can vary greatly. Dry air contains much less water vapor than the air above a rainforest. The amount of water vapor in the air is called humidity. Warm, tropical air tends to contain more water vapor than cold, polar air does.
When you hear someone talking about the “humidity” of the air, that person is probably talking about relative humidity. Relative humidity is the ratio of the amount of water vapor in the air to the maximum amount of water vapor that can exist at that temperature.
Relative humidity is always expressed as a percentage. When the relative humidity is 60 percent, for example, air contains 60 percent of the maximum amount of water vapor that can exist at that temperature. When relative humidity reaches 100 percent, air is said to be saturated.
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