Scaffolded notes
Build the idea
Air is a mixture
Dry air near Earth's surface is about 78 percent nitrogen, 21 percent oxygen, and almost 1 percent argon. Carbon dioxide and other gases occur in much smaller amounts. A mixture keeps the properties of its parts; the gases are not joined into one new substance.
Water vapor changes
Water vapor commonly ranges from nearly 0 to about 4 percent of air. Warm tropical air can contain much more water vapor than cold, dry polar air. Water vapor provides material for clouds and precipitation and is also a greenhouse gas.
Small does not mean unimportant
Ozone blocks much harmful ultraviolet radiation in the stratosphere. Carbon dioxide, methane, and water vapor absorb some outgoing infrared energy. Scientists monitor these trace gases because changes in atmospheric composition can affect living things and Earth's energy balance.
Stop and explain
Check your thinking
1. What percentage of dry air is nitrogen plus oxygen?
Possible response: About 99 percent.
2. Why might two air samples have different water-vapor percentages?
Possible response: Humidity, temperature, location, and recent weather differ.
Interactive challenge
Try the science
Match each atmospheric component to its best description.
Before you leave
Exit ticket
Explain why a trace gas can still influence Earth systems.