I Clouds are formed as warm, moist air is forced upward, expands, and cools.
I An air mass is stable if it tends to return to its original height after it starts rising.
I Cloud droplets form when water vapor is cooled to the dew point and condenses on condensation nuclei.
I Clouds are classified by their shapes and the altitudes at which they form.
I Cloud droplets collide and coalesce into larger droplets that can fall to Earth as rain, snow, sleet, or hail.
Understand Main Ideas
1. iman<TTflH Summarize the differences between low clouds, middle clouds, and high clouds.
2. Describe how precipitation forms.
3. Determine the reason precipitation will fall as snow rather than rain.
4. Compare stable and unstable air. Think Critically
5. Evaluate how a reduction in the number of condensation nuclei in the troposphere would affect precipitation. Explain your reasoning.
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6. Describe the path a drop of rain might follow throughout the water cycle.
Self-Check Quiz glencoe.com
Section 3 • Clouds and Precipitation 303
Atoms, such as chlorine and bromine, when located in the stratosphere, can destroy ozone molecules. The decline in stratospheric ozone measured since the early 1980s might have been reversed due to a decrease in stratospheric chlorine.
Variations in ozone amounts The total amount of ozone in the atmosphere over Earth's surface varies with location and also changes with time. Total ozone increases with latitude, being low at the equator and highest in the polar regions. Ozone amounts also vary seasonally, usually decreasing from winter to summer. The largest seasonal changes occur at high latitudes, particularly in the polar regions.
The Antarctic ozone hole Over Antarctica, the lowest ozone amounts occur in early spring. Since the late 1970s, total ozone amounts in the spring have greatly decreased.
This decrease in springtime ozone over Antarctica is called the Antarctic ozone hole. The Antarctic ozone hole is caused by chlorofluorocarbons (CFCs) and the presence of polar stratospheric clouds (PSCs). These clouds form over Antarctica during the winter in the lower stratosphere. CFCs break down, producing molecules that contain chlorine atoms. These molecules undergo chemical reactions on ice crystals in the PSCs, producing chlorine and other compounds that destroy ozone.
The Montreal Protocol Satellite measurements beginning in the late 1970s also showed a decrease in global ozone amounts of several percent. Concerns over decreasing ozone led to the adoption of the Montreal Protocol in 1987. This international agreement requires countries to phase out the production and use of CFCs and similar chemicals. As a result, levels of chlorine and other ozone-destroying chemicals in the stratosphere have been declining since the late 1990s, as shown in the graph.
Signs of recovery? Between 1996 and 2006, the decrease in total ozone leveled off in most regions. Part of these changes might be due to natural causes, such as solar variability, as well as to the Montreal Protocol. Measurements over several more years will be needed to determine whether the ozone decline has been reversed.
Earth Science ^
Magazine Article Research how natural processes, such as volcanic eruptions, solar activity, and air movements affect ozone levels in the stratosphere. Write a magazine article that reports what you found. To learn more about the ozone layer, visit glencoe.com.
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