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.

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).

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

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).

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.

Troposphere

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.

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.

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.”

Stratosphere

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.

Mesosphere

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.

Thermosphere

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.

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.

• aurora: A bright and ever-changing display of light caused by solar radiation interacting with the upper atmosphere in the region of the poles. It is called aurora borealis in the Northern Hemisphere and aurora australis in the Southern Hemisphere.

• environmental lapse rate: The rate of temperature decrease with height in the troposphere.

• ionosphere: A complex atmospheric zone of ionized gases extending between 80 and 400 kilometers, thus coinciding with the lower thermosphere and heterosphere.

• mesopause: The boundary between the mesosphere and the thermosphere.

• mesosphere: The zone in the atmosphere above the stratosphere that is characterized by temperatures decreasing with height.

• radiosonde: A lightweight package of weather instruments fitted with a radio transmitter and carried aloft by a balloon.

• stratopause: The boundary between the stratosphere and the mesosphere.

• stratosphere: The zone of the atmosphere above the troposphere characterized at first by isothermal conditions and then a gradual temperature increase. Earth’s ozone is concentrated here.

• temperature inversion: A layer in the atmosphere of limited depth where the temperature increases rather than decreases with height.

• thermosphere: The zone of the atmosphere beyond the mesosphere in which there is a rapid rise in temperature with height.

• tropopause: The boundary between the troposphere and the stratosphere.

• troposphere: The lowermost layer of the atmosphere, marked by considerable turbulence and, in general, a decrease in temperature with increasing height.

• Pressure is the weight of the air above a location. Because air is compressible, pressure decreases at an increasing rate as you go up in the atmosphere.

• Based on temperature, the atmosphere is divided vertically into four layers. The troposphere is the lowermost layer. In the troposphere, temperature usually decreases with increasing altitude. Essentially, all important weather phenomena occur in the troposphere.

• Above the troposphere is the stratosphere, which warms with altitude because of absorption of UV radiation by ozone. In the mesosphere, temperatures again decrease. Upward from the mesosphere is the thermosphere, a layer with only a tiny fraction of the atmosphere’s mass and no well-defined upper limit.

• The ionosphere is an electrically charged layer of the atmosphere where molecules of nitrogen and atoms of oxygen are readily ionized as they absorb solar radiation.

• Auroras (the northern and southern lights) occur within the ionosphere. Auroras form as atomic particles ejected from the Sun during solar flare activity enter the atmosphere near Earth’s magnetic poles and energize the atoms of oxygen and molecules of nitrogen, causing them to emit light.