Composition and Structure of the Atmosphere

Composition and Structure of the Atmosphere

do you see when you look at the sky? You might see clouds, blue sky, lightning, or even a haze of air pollution. All of these things are in the atmosphere, a layer of gases that surrounds Earth.
Whats in the air?
The atmosphere is mostly air, which is made of gas molecules. As shown in Figure 1, about four out of five molecules in the air are nitrogen gas (N2). About one molecule out of five is oxygen gas (O2).
There are also small amounts of other gases such as water vapor, argon (Ar) and carbon dioxide (CO2). Besides air, the atmosphere also contains tiny solid particles such as dust and pollen. Some of the particles in the atmosphere even come from volcanic eruptions.
1. Key Concept CheckWhat is the composition of the atmosphere?
Figure 1 Air mostly is made of nitrogen and oxygen. There are also small amounts of other gases in the air.
Layers of the Atmosphere
The characteristics and composition of Earths atmosphere change with altitude. Molecules at the bottom of the atmosphere are packed together more closely than molecules higher in the atmosphere. This is because the weight of all the molecules above pushes down on the molecules at the bottom of the atmosphere. Air pressure is the pressure that a column of air exerts on the air, or the surface, below it. When molecules in the air are packed closely together, it has high pressure. When molecules in the air are more spread out, it has low pressure.
Scientists divide the atmosphere into layers that have distinct characteristics. You can see these layers in Figure 2. The troposphere (TRO puh sfihr) is the atmospheric layer closest to the Earth extending from Earths surface to about 10 km above it.
2. Key Concept CheckWhat is the structure of the atmosphere?
Figure 2 Scientists divide the atmosphere into five layers that have special characteristics. In some layers, temperature increases with altitude, but temperature decreases in other layers.
Visual CheckWhich layer contains the ozone layer?
Moving Air
Air pressure in the troposphere is always changing because air in the troposphere is always moving. Changing air pressures create wind patterns and cause weather events.
The Importance of the Sun
The Sun warms the rocks, the soil, and the water at Earths surface. In turn, the Earth warms the air in the troposphere. Because warm air is less dense than cool air, it rises high in the troposphere. As the air moves higher, it cools. Denser, cooled air sinks. It flows toward the low-pressure area left by the rising warm air. The cooled air then warms and rises again. The circulation of rising, less dense, warm air and sinking, more dense, cool air is called convection. As illustrated in Figure 3, convection is responsible for the movement of air in the troposphere.
1. Key Concept CheckWhat causes air to move ?
Figure 3 Cool air flows into a lower pressure area forcing the warmer air upward. After the air cools, it sinks toward Earth. As hot air rises, cool air replaces it.
Local Winds
Some winds blow over short distances. A local wind results from air flowing from an area of higher air pressure to an area of lower air pressure. Differences in pressure result when the atmosphere is warmer in one area than in another.
Global Winds
At Earths surface, atmospheric convection makes giant bands of winds, as shown in Figure 4. The westerlies generally blow from west to east, but Earths rotation causes them to turn away from the equator. The trade winds generally blow east to west, but Earths rotation causes them to turn toward the equator.
Figure 4 Earths rotation affects the direction of global winds. The westerlies blow from southwest to northeast in the northern hemisphere and from northwest to southeast in the southern hemisphere.
from Latin convectionem, meansthe act of carrying
Have you ever driven up a mountain into a cloud? A cloud is a group of water droplets or ice crystals so small that they float in the air. Clouds are important because they transport water to different areas of Earth.
Cloud Formation
How does the water vapor that makes clouds get into the sky? Most of it gets there by evaporation. Then, through the process of condensation, water vapor turns into clouds.
Evaporation The process of a liquid, such as water, changing into a gas, such as water vapor, is called evaporation. Thermal energy, usually from sunlight, heats water and causes it to evaporate. Water evaporates from the surfaces of bodies of water, such as the ocean, lakes, and rivers and from organisms, such as trees and humans.
Condensation The process of water vapor gas changing into liquid water is condensation. Water vapor condenses around tiny particles of dust, pollen, or other air pollution. These water droplets are so small that they are able to float in the air. Many billions of these little water droplets make up a cloud. As more water condenses and the droplets increase in size, they can become so large that they fall from the sky as rain or snow.
1. Key Concept CheckHow do clouds form?
Types of Clouds
There are many types of clouds. Each type has a specific appearance and forms at a specific height in the troposphere. The three main cloud types are shown in Figure 5.
Cumulus Clouds Puffy clouds that look like cotton balls are called cumulus (KYEW myuh lus) clouds. The bases of these clouds are usually flat. Some cumulus clouds are small and low. Others grow taller and larger until they reach the top of the troposphere. These are the kind of clouds that make thunderstorms.
Stratus Clouds It might be hard for you to see individual stratus clouds or find the edges of them. They usually spread across most or all of the visible sky. Stratus clouds are low in the sky and can produce drizzle or light rain.
Cirrus Clouds High in the troposphere is where cirrus (SIHR us) clouds form. They are made of ice crystals that fan out in the wind and create long, wispy shapes. Cirrus clouds sometimes are called mares tails because they look like a horses tail. Cirrus clouds do not produce rain or snow.
2. Key Concept CheckWhat are the three main types of clouds?
(l) MIMOTITO/Getty Images, (c) Brian Cosgrove/Dorling Kindersley, (r) age fotostock/SuperStock
Figure 5 Cumulus, cirrus, and stratus clouds are recognizable by their different shapes. Can you find any of these cloud types in the sky today?
Visual CheckWhich type of cloud looks like a horses tail?
Lesson Review
Visual Summary
The atmosphere is divided into five layers. The air in the atmosphere is mostly nitrogen and oxygen. The air also contains argon, carbon dioxide and water vapor.
Atmospheric convection in the troposphere makes bands of winds that blow worldwide.
There are three main types of clouds: cirrus, stratus, and cumulus.
MIMOTITO/Getty Images
Lesson Assessment
Use Vocabulary
1. The__________is the atmospheric layer closest to Earth’s surface.
2. The force exerted by air is called__________.
air pressure
Understand Key Concepts
3. Describe how Earth’s rotation affects global winds.
Earth’s rotation causes global winds to turn to the right in the northern hemisphere and to the left in the southern hemisphere.
4. Which cloud is associated with thunderstorms?
5. Compare the characteristics of the troposphere and the stratosphere.
The troposphere has higher air pressure, is where weather happens and where clouds form, and gets colder the higher you go. The stratosphere has lower air pressure, includes the ozone layer, and gets warmer the higher you go.
6. Weather happens in which layer of the atmosphere?
7. Local winds blow from area of high pressure to an area of low pressure. area of high pressure to another area of high pressure. area of low pressure to an area of high pressure. area of low pressure to another area of low pressure.
8. Which type of cloud is shown below?
Photograph of a large, dark cloud raining on the land below.
Steve Allen/Getty Images
B. cumulus
C. mare's tail
9. Which process forms the droplets that make up clouds?

Interpret Graphics
10. Summarize how temperature changes through the layers of the atmosphere using the table below.

Example of a two-column table used to organize information about the layers of the atmosphere
exosphere—very hot; thermosphere—temperature increases with height; mesosphere—temperature decreases with height; stratosphere—temperature increases with height; troposphere—temperature decreases with height
11. Describe what causes air to flow in the pattern of convection shown here.
Diagram of convection with arrows moving in a clockwise direction; blue arrows are poining downward, and red arrows are pointing upward.
Warm air rises up, cools, flows down, and then is warmed again.
Critical Thinking
12. Decide Where on Earth is the convection cycle the strongest?
The convection cycle would be the strongest at the equator, where the Sun warms Earth’s surface the most.
13. Hypothesize how an increase in air pollution might affect cloud formation.
Because water droplets that make up clouds form on air pollution particles, an increase in air pollution could lead to more clouds.
14. Synthesize How would holes in the ozone layer affect life on Earth?
Holes in the ozone layer would mean less protection for life on Earth from the Sun’s rays.
15. Hypothesize How would global winds be different if Earth did not rotate?
Sample answers: Winds would not turn right in the northern hemisphere and left in the southern hemisphere. Without spin, one side of Earth would have day and the other side night, so equal warming wouldn’t happen around the whole equator every 24 hours like it does now.
16. Infer Where do more clouds form—over a desert or over the ocean?


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