### Lesson Objectives

• Describe how the temperature of the atmosphere changes with altitude.
• Outline the properties of the troposphere.
• Explain the role of the ozone layer in the stratosphere.
• Describe conditions in the mesosphere.
• Explain how the sun affects the thermosphere.
• Identify the exosphere.

### Vocabulary

• exosphere
• mesosphere
• ozone
• stratosphere
• temperature inversion
• thermosphere
• troposphere

### Introduction

Earth’s atmosphere is divided into five major layers. The layers are based on temperature.

### Temperature of the Atmosphere

Air temperature changes as altitude increases. In some layers of the atmosphere, the temperature decreases. In other layers, it increases. You can see this in Figure below and at the link below. Refer to the figure as you read about the layers below.

How does air temperature change in the layer closest to Earth?

### Troposphere

The troposphere is the lowest layer of the atmosphere. In it, temperature decreases with altitude. The troposphere gets some of its heat directly from the sun. Most, however, comes from Earth's surface. The surface is heated by the sun and some of that heat radiates back into the air. This makes the temperature higher near the surface than at higher altitudes.

#### Properties of the Troposphere

Look at the troposphere in Figure above. This is the shortest layer of the atmosphere. It rises to only about 12 kilometers (7 miles) above the surface. Even so, this layer holds 75 percent of all the gas molecules in the atmosphere. That’s because the air is densest in this layer.

#### Mixing of Air

Air in the troposphere is warmer closer to Earth’s surface. Warm air is less dense than cool air, so it rises higher in the troposphere. This starts a convection cell. Convection mixes the air in the troposphere. Rising air is also a main cause of weather. All of Earth’s weather takes place in the troposphere.

#### Temperature Inversion

Sometimes air doesn’t mix in the troposphere. This happens when air is cooler close to the ground than it is above. The cool air is dense, so it stays near the ground. This is called a temperature inversion. An inversion can trap air pollution near the surface. Temperature inversions are more common in the winter. Can you explain why?

Temperature Inversion and Air Pollution. How does a temperature inversion affect air quality?

#### Tropopause

At the top of the troposphere is a thin layer of air called the tropopause. You can see it in Figure above. This layer acts as a barrier. It prevents cool air in the troposphere from mixing with warm air in the stratosphere.

### Stratosphere

The stratosphere is the layer above the troposphere. The layer rises to about 50 kilometers (31 miles) above the surface.

#### Temperature in the Stratosphere

Air temperature in the stratosphere layer increases with altitude. Why? The stratosphere gets most of its heat from the sun. Therefore, it’s warmer closer to the sun. The air at the bottom of the stratosphere is cold. The cold air is dense, so it doesn’t rise. As a result, there is little mixing of air in this layer.

#### The Ozone Layer

The stratosphere contains a layer of ozone gas. Ozone consists of three oxygen atoms (O3). The ozone layer absorbs high-energy UV radiation. As you can see in Figure below, UV radiation splits the ozone molecule. The split creates an oxygen molecule (O2) and an oxygen atom (O). This split releases heat that warms the stratosphere. By absorbing UV radiation, ozone also protects Earth’s surface. UV radiation would harm living things without the ozone layer.

How does the ozone layer protect Earth

#### Stratopause

At the top of the stratosphere is a thin layer called the stratopause. It acts as a boundary between the stratosphere and the mesosphere.

### Mesosphere

The mesosphere is the layer above the stratosphere. It rises to about 85 kilometers (53 miles) above the surface. Temperature decreases with altitude in this layer.

#### Temperature in the Mesosphere

There are very few gas molecules in the mesosphere. This means that there is little matter to absorb the sun’s rays and heat the air. Most of the heat that enters the mesosphere comes from the stratosphere below. That’s why the mesosphere is warmest at the bottom.

#### Meteors in the Mesosphere

Did you ever see a meteor shower, like the one in Figure below? Meteors burn as they fall through the mesosphere. The space rocks experience friction with the gas molecules. The friction makes the meteors get very hot. Many meteors burn up completely in the mesosphere.

Friction with gas molecules causes meteors to burn up in the mesosphere.

#### Mesopause

At the top of the mesosphere is the mesopause. Temperatures here are colder than anywhere else in the atmosphere. They are as low as -100° C (-212° F)! Nowhere on Earth’s surface is that cold.

### Thermosphere

The thermosphere is the layer above the mesosphere. It rises to 600 kilometers (372 miles) above the surface. The International Space Station orbits Earth in this layer as in Figure below.

The International Space Station orbits in the thermosphere.

#### Temperature in the Thermosphere

Temperature increases with altitude in the thermosphere. Surprisingly, it may be higher than 1000° C (1800° F) near the top of this layer! The sun’s energy there is very strong. The molecules absorb the sun's energy and are heated up. But there are so very few gas molecules, that the air still feels very cold. Molecules in the thermosphere gain or lose electrons. They then become charged particles called ions.

#### Northern and Southern Lights

Have you ever seen a brilliant light show in the night sky? Sometimes the ions in the thermosphere glow at night. Storms on the sun energize the ions and make them light up. In the Northern Hemisphere, the lights are called the northern lights, or aurora borealis. In the Southern Hemisphere, they are called southern lights, or aurora australis.

Glowing ions in the thermosphere light up the night sky.

### Exosphere

The exosphere is the layer above the thermosphere. This is the top of the atmosphere. The exosphere has no real upper limit; it just gradually merges with outer space. Gas molecules are very far apart in this layer, but they are really hot. Earth’s gravity is so weak in the exosphere that gas molecules sometimes just float off into space.

### Lesson Summary

• Earth’s atmosphere is divided into five major layers. The layers are based on temperature.
• The troposphere is the lowest layer. Temperature decreases with altitude in this layer. All weather takes place here.
• The stratosphere is the layer above the troposphere. Temperature increases with altitude in this layer. The ozone layer occurs here.
• The mesosphere is the layer above the stratosphere. Temperature decreases with altitude in this layer. Meteors burn up here.
• The thermosphere is the layer above the mesosphere. Temperature increases with altitude in this layer. The northern and southern lights occur here.
• The exosphere is the highest layer. Air molecules are very far apart. They may escape Earth’s gravity and float into space.

### Lesson Review Questions

#### Recall

1. How does temperature change in the troposphere?

2. What is a temperature inversion?

3. Why is the ozone layer in the stratosphere important to life on Earth?

4. Where does the mesosphere get its heat?

#### Apply Concepts

5. Think of a creative way you could model the layers of the atmosphere. Describe your model. How does it show temperature differences between the layers?

#### Think Critically

6. How is a temperature inversion like the temperatures of the stratosphere and troposphere?

7. Explain why air mixes in the troposphere but not in the stratosphere.

8. Why is there a hole in the ozone layer? What do you think the consequences of that hole are?

### Points to Consider

Energy from the sun is responsible for winds that blow in the troposphere.

• What is wind?
• How does energy cause winds to blow?