Section 1.5:
Vertical Structure of the Atmosphere

Learning Objective

Interpret a graph that shows changes in air pressure from Earth’s surface to the top of the atmosphere. Sketch and label a graph that shows the thermal structure of the atmosphere.

Section Content

Mini-Lecture Video - Extent of the Atmosphere (Click to watch the video)

When compared to the size of the solid Earth, the envelope of air surrounding our planet is indeed very shallow. To say that the atmosphere begins at Earth’s surface and extends upward is obvious. However, where does the atmosphere end, and where does outer space begin? There is no sharp boundary; the atmosphere rapidly thins as you travel away from Earth, until there are too few gas molecules to detect.

Pressure Changes

To understand the vertical extent of the atmosphere, let us examine the changes in atmospheric pressure with height. Atmospheric pressure is simply the weight of the air above. To describe atmospheric pressure, the National Weather Service uses a measure called the millibar (mb), which will be discussed in detail in Chapter 6. At sea level, the average pressure is slightly more than 1000 millibars. This corresponds to a weight of about 14.7 pounds per square inch. Obviously, the pressure at higher altitudes is less because there is less air (fewer air molecules) above these altitudes (Figure 1.18).

Figure 1.18
Air pressure changes with altitude

The rate of pressure decrease with an increase in altitude is not constant. Pressure decreases rapidly near Earth’s surface and more gradually at greater heights. Put another way, the figure shows that the vast bulk of the gases making up the atmosphere is near Earth’s surface and that the gases gradually merge with the emptiness of space.

One-half of the atmosphere lies below an altitude of 5.6 kilometers (3.5 miles). At about 16 kilometers (10 miles), 90 percent of the atmosphere has been traversed. At an altitude of 100 kilometers, the atmosphere is so thin that the density of air is less than could be found in the most perfect artificial vacuum at the surface. Nevertheless, the atmosphere continues to even greater heights. In fact, traces of our atmosphere extend for thousands of kilometers beyond Earth’s surface. Thus, to say where the atmosphere ends and outer space begins is arbitrary and depends on what phenomenon one is studying. It is apparent that there is no sharp boundary.

The graphic portrayal of pressure data in Figure 1.18 shows that the rate of pressure decrease is not constant. Rather, air pressure falls at a decreasing rate with an increase in altitude. Put another way, air is highly compressible—that is, the gases that make up air expand with decreasing pressure and become compressed with increasing pressure.

Temperature Changes

Mini-Lecture Video - Temperature Structure of the Atmosphere (Click to watch the video)

By the early twentieth century, scientists collecting data obtained from balloons and kites found that the air temperature dropped with increasing height above Earth’s surface. This phenomenon is felt by anyone who has climbed a high mountain and is obvious in pictures of snow-capped mountaintops rising above snow-free lowlands (Figure 1.19).

Figure 1.19
Temperature change in the troposphere

Snow-capped mountains and snow-free lowlands are a reminder that temperatures decrease as we go higher in the troposphere.

Scientists once believed that the temperature continued to decrease with height to a value of absolute zero (–273°C) at the outer edge of the atmosphere. In 1902, however, French scientist Leon Philippe Teisserenc de Bort refuted the notion that temperature decreases continuously with an increase in altitude. In studying the results of more than 200 balloon launchings, Teisserenc de Bort found that the temperature leveled off at an altitude between 8 and 12 kilometers (5 and 7.5 miles). Later, the use of balloons and rocket-sounding techniques revealed the temperature structure of the atmosphere up to great heights. Based on these temperature measurements, the atmosphere can be divided vertically into four layers (Figure 1.20). The temperature profile shown in Figure 1.20 represents the average temperature change with altitude. However, the actual temperature profile can be quite variable from one day to the next—particularly in the lower atmosphere.

Figure 1.20
Thermal structure of the atmosphere

Earth’s atmosphere is traditionally divided into four layers, based on temperature.


The bottom layer in which we live, where average temperatures decrease with an increase in altitude, is the troposphere. The term was coined in 1908 by Teisserenc de Bort and literally means the region where air “turns over,” a reference to the appreciable vertical mixing of air in this lowermost zone.

The temperature decrease in the troposphere is called the environmental lapse rate. Its average value is 6.5°C per kilometer (3.5°F per 1000 feet), a figure known as the normal lapse rate. It should be emphasized, however, that the environmental lapse rate is not a constant but rather can be highly variable and must be regularly measured. Radiosondes are used to measure the actual environmental lapse rate, as well as gather information about vertical changes in air pressure, wind, and humidity. A radiosonde is an instrument package that is attached to a balloon and transmits data by radio as it ascends through the atmosphere (Figure 1.21). The environmental lapse rate can vary over the course of a day as a result of fluctuations in weather, as well as seasonally and from place to place. Sometimes shallow layers where temperatures actually increase with height are observed in the troposphere. Such reversals, called temperature inversions, are described in greater detail in Chapter 13.

Figure 1.21

This lightweight package of instruments is carried aloft by a small weather balloon. It transmits data on vertical changes in temperature, pressure, and humidity in the troposphere. The troposphere is where practically all weather phenomena occur, so it is very important to have frequent measurements.

The temperature continues to decrease to an average height of about 12 kilometers (7.5 miles), which marks the top of the troposphere, called the tropopause (see Figure 1.20). Yet the thickness of the troposphere is not the same everywhere. In the tropics, the tropopause reaches heights in excess of 16 kilometers (10 miles), whereas in polar regions it is lower, varying from about 7 to 8 kilometers (about 5 miles) (Figure 1.22). Warm surface temperatures and highly developed thermal mixing as the warmed air rises are responsible for the greater vertical extent of the troposphere near the equator.

Figure 1.22
Differences in the height of the tropopause

The variation in the height of the tropopause, as shown on the small inset diagram, is greatly exaggerated.

The troposphere is the chief focus of meteorologists because it is in this layer that essentially all important weather phenomena occur. Almost all clouds and certainly all precipitation, as well as all our violent storms, are born in this lowermost layer of the atmosphere. This is why the troposphere is often called the “weather sphere.”


Above the troposphere lies the stratosphere. In the stratosphere, the temperature at first remains nearly constant to a height of about 20 kilometers (12 miles) before it begins a sharp increase that continues until the stratopause is encountered at a height of about 50 kilometers (30 miles) above Earth’s surface (see Figure 1.20). The high concentration of ozone in the stratosphere accounts for the rise in temperature observed in this layer. Recall that ozone absorbs ultraviolet radiation from the Sun, which in turn causes its temperature to rise.

Although the troposphere is dominated by large-scale turbulence and mixing, very little vertical mixing occurs in the stratosphere. This is because the stratosphere experiences a temperature inversion, where cold air lies beneath warm air, in contrast to the opposite occurrence in the troposphere.


In the third layer, the mesosphere, temperatures decrease with height until the mesopause, or top of the mesosphere, is reached (see Figure 1.20). This decrease in temperature with height leads to abundant vertical mixing. The mesopause is located about 80 kilometers (50 miles) above the surface, where the average temperature approaches a chilly −90°C (−130°F)—the coldest temperatures anywhere in the atmosphere.

The mesosphere is one of the least explored regions of the atmosphere because it cannot be reached by the highest-flying airplanes and research balloons, nor is it accessible to the lowest-orbiting satellites. Recent technical developments are just beginning to fill this knowledge gap.


The fourth layer extends outward from the mesopause and has no well-defined upper limit. It is the thermosphere, a layer that contains only a tiny fraction of the atmosphere’s mass. In the extremely rarified air of this outermost layer, temperatures again increase as oxygen and nitrogen atoms absorb very shortwave, high-energy solar radiation (Figure 1.20).

Temperatures rise to extremely high values of more than 1000°C (1800°F) in the thermosphere. But such temperatures are not comparable to those experienced near Earth’s surface. Temperature is defined in terms of the average speed at which molecules move—the higher the speed, the higher the temperature. Because the gases of the thermosphere are moving at very high speeds, the temperature is very high. But the gases are so sparse that collectively they possess only an insignificant quantity of thermal energy (heat). For this reason, the temperature of a satellite orbiting Earth in the thermosphere is determined chiefly by the amount of solar radiation it absorbs, and not by the high temperature of the almost nonexistent surrounding air. If an astronaut inside were to expose his or her hand, the air in this layer would not feel hot.

The Ionosphere

In addition to the layers defined by vertical variations in temperature, scientists recognize another layer in the atmosphere. Located between 80 and 400 kilometers (50 to 250 miles) above Earth’s surface, and thus coinciding with the lower portion of the thermosphere, is an electrically charged layer known as the ionosphere. Here molecules of nitrogen and atoms of oxygen are readily ionized as they absorb high-energy shortwave solar radiation. Ionization is a process in which the affected molecule or atom loses one or more electrons and becomes a positively charged ion, and the electrons set free then travel as electric currents.

As best we can tell, the ionosphere has little impact on our daily weather. But this layer of the atmosphere is the site of one of nature’s most interesting spectacles, the auroras (Figure 1.23). The aurora borealis (northern lights) and its Southern Hemisphere counterpart, the aurora australis (southern lights), appear in a wide variety of forms. Sometimes the displays consist of vertical streamers in which there can be considerable movement. At other times, the auroras appear as a series of luminous expanding arcs or as a quiet glow that has an almost foglike quality.

Figure 1.23
The auroras

The aurora borealis (northern lights), as seen in Alaska. The same phenomenon occurs toward the South Pole, where it is called the aurora australis (southern lights).

Auroral displays are aligned with Earth’s magnetic poles and closely correlated with large solar storms, such as solar flares. Solar flares are massive magnetic storms on the Sun that emit enormous quantities of fast-moving atomic particles. As these charged particles (ions) approach Earth, they are captured by its magnetic field, which in turn guides them toward the magnetic poles. Then, as the ions impinge on the ionosphere, they energize the atoms of oxygen and molecules of nitrogen and cause them to emit light—the glow of the auroras. Because the occurrence of solar storms is closely associated with sunspot activity, auroral displays increase conspicuously at times when sunspots are most numerous.

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

Does air pressure increase or decrease with an increase in altitude? Is the rate of change constant or variable? Explain.

Decrease, but the rate of pressure decrease is not constant. Air is highly compressible and pressure decreases at a decreasing rate with an increase in altitude until, beyond an altitude of about 3 kilometers (22 miles), the decrease is negligible. 

The atmosphere is divided vertically into four layers based on temperature. List these layers in order from lowest to highest. In which layer does practically all weather occur?

The lowermost layer of the atmosphere is the troposphere. It is in the troposphere that practically all our weather occurs. The next layer is the stratosphere, followed by the mesosphere, and finally the thermosphere.

What is the ionosphere? How is it related to the auroras?

The ionosphere is an electrically charged layer located in the altitude range between 80 and 400 kilometers (50 to 250 miles), where molecules of nitrogen and atoms of oxygen are readily ionized as they absorb high-energy shortwave solar energy. Clouds of charged particles are emitted during magnetic storms on the Sun; as they approach Earth, they are captured by Earth’s magnetic field, which guides them toward the magnetic poles. As the ions enter Earth's upper atmosphere near the poles, they energize atoms of oxygen and molecules of nitrogen, causing them to emit light.