Name and describe the atmosphere’s layers.

As we will see throughout this book, many of Earth’s physical systems are composed of layers. In the lithosphere, layers make up Earth’s internal structure. In the hydrosphere, the oceans have layers distinguished by light penetration, temperature, and life. In tropical rainforests, vegetation forms many different canopy and understory layers.

The atmosphere, too, is structured in layers. The homosphere and the heterosphere layers of the atmosphere can be distinguished based on concentrations of gases. These layers, however, play a minor role in Earth’s physical systems. Layers based on changing trends in temperature with altitude, however, have important effects on Earth’s physical systems.

You may be familiar with the way the temperature of the atmosphere changes with altitude. If you have ever been high in the mountains, you probably noticed that the air is cooler there than at lower elevations. But if you kept climbing higher in the atmosphere, you might be surprised to find that high above the highest mountains, the atmosphere gets warmer again.

There are four main thermal divisions of the atmosphere based on changes in temperature with altitude: The troposphere extends from Earth’s surface up to about 12 km (7.5 mi), on average. It is the lowest layer of the atmosphere, in which all weather occurs. The next layer up, the stratosphere, is found between 12 and 50 km (7.5 and 30 mi) above Earth’s surface. The stratosphere contains a permanent temperature inversion because it is heated as it absorbs radiation from the Sun. A temperature inversion is a layer of the atmosphere in which air temperature increases with increased height (see also Section 1.4). Above the stratosphere, the mesosphere lies between 50 and 80 km (30 and 50 mi) above Earth’s surface. The thermosphere is located from 80 to 600 km (50 to 370 mi) above Earth’s surface. Figure 1.5 illustrates these divisions and their main characteristics.

41

Our knowledge of the atmosphere’s structure is less than a century old. Much of what scientists know about the atmosphere comes from two main sources of information: weather balloons and orbiting satellites. The development of this knowledge is sketched in Figure 1.6.

The troposphere is warmed from the bottom up by Earth’s surface, which is warmed by sunlight. As one moves higher in the troposphere, air temperature generally decreases. The environmental lapse rate is the rate of cooling with increasing altitude in the troposphere. The average environmental lapse rate is 6.5°C per 1,000 m or 3.6°F per 1,000 ft. Keep in mind that this is an average rate, and that the atmosphere may cool much faster or slower, depending on the time and the location.

42

In the Crunch the Numbers feature above, you will use the environmental lapse rate to calculate the temperature you can expect on a mountaintop.

In the lower troposphere, temperature inversions sometimes develop. There are many ways in which temperature inversions may form. They often develop in valleys, where a cold, dense mass of air may form close to the ground. Above this cold air mass, the temperature increases. Temperature inversions in the troposphere are usually no more than a few hundred meters thick, and a little higher up, the normal cooling trend resumes.

Temperature inversions in the atmosphere are significant in several ways. The temperature inversion in the stratosphere limits almost all clouds to the troposphere, as we’ll see later in this section. Temperature inversions in the troposphere can trap pollutants near the ground surface (see Section 1.4). They also can create sleet and freezing rain during winter storms.

Roughly 80% of the mass of the atmosphere is in the troposphere, and it is where Earth’s weather occurs.Almost all clouds, and all storms and precipitation, are limited to the troposphere. The troposphere experiences strong vertical mixing: The Greek word tropos means to turn over, or convect. The troposphere ends, and the stratosphere begins, at the tropopause. Almost all cloud tops end at the tropopause, where the atmosphere begins to warm due to the temperature inversion in the stratosphere. The temperature inversion prevents clouds from rising, as discussed in the Picture This feature below.

As we have seen, the average height of the tropopause is about 12 km (7.5 mi). At the equator, the tropopause is 18 km (11 mi) above Earth’s surface. At midlatitudes, it is about 12 km (7.5 mi) above Earth’s surface, and in the polar regions it occurs at about 8 km (5 mi) in altitude (Figure 1.7).

Why is the tropopause roughly twice as high over the equator as over the poles? There are two reasons. First, solar heating of the atmosphere expands air and causes it to occupy more space. Heating is greatest near the equator, in the tropics. Second, Earth rotates faster at the equator than in polar regions (see Section GT.2). Therefore, the atmosphere near the equator bulges outward more than that at higher latitudes, increasing its depth. The height of the tropopause also increases in summer and decreases in winter due to changing temperatures in the atmosphere.

43

Above the tropopause is the stratosphere. There are very few clouds in the stratosphere, and the air is mostly free of atmospheric aerosols. As Figure 1.8 shows, the contrast between the two layers can be vivid.

As a result of its permanent temperature inversion, there is little vertical mixing in the stratosphere—air flows in horizontal sheets instead—and storms and clouds usually do not enter the stratosphere from below. The temperature inversion in the stratosphere is caused by the absorption of ultraviolet radiation by stratospheric ozone in the ozonosphere. Ultraviolet (UV) radiation is solar radiation that has shorter wavelengths than visible light. It can cause skin cancer and cataracts in people. The ozonosphere is a region of the stratosphere with high concentrations of ozone molecules that block UV radiation. The critical role of the ozonosphere is explored further in the Geographic Perspectives at the end of this chapter. The top of the stratosphere, the stratopause, occurs at about 50 km (30 mi) in altitude.

44

The mesosphere lies above the stratosphere. Although air density (and pressure) in the mesosphere is extremely low, meteors (sometimes called shooting stars) heat up and vaporize as they encounter friction with the air molecules in the mesosphere. As in the troposphere, temperature decreases with height in the mesosphere, until it reaches about −90°C (−130°F) at about 85 km (53 mi) above Earth’s surface.

Above the mesopause lies the thermosphere. As in the stratosphere, there is a permanent temperature inversion in the thermosphere, caused by absorption of the Sun’s radiation by gas molecules. Temperatures in the thermosphere rise as high as 1,200°C (2,200°F). What would the thermosphere feel like to a person? The air would feel cold because it is extremely thin (air molecules are exceedingly far apart). As a result, very little heat would be transferred to your body.

The top of the thermosphere (the thermopause) is 600 km (370 mi) above Earth’s surface. Here, the air is so thin that molecules move some 10 km (6 mi), on average, before colliding with another molecule. In the troposphere, where molecular density is much greater, molecules move only a millionth of a centimeter, on average, before colliding. At the top of the thermosphere some molecules reach escape velocity, meaning that they can overcome Earth’s gravitational pull and enter the depths of space forever. The region where molecules are free of Earth’s gravity is the exosphere.

The last atmospheric layer we will examine, the ionosphere, is distinguished by its electrical properties. The ionosphere is a region of the upper mesosphere and the thermosphere between roughly 80 and 600 km (50 to 373 mi) in altitude where nitrogen and oxygen are ionized by solar energy. The ionosphere interacts with and absorbs UV, X-ray, and gamma radiation from the Sun, all of which can harm life on Earth. The ionosphere also allows radio waves transmitted from Earth’s surface to travel long distances through the atmosphere and is therefore important to radio communications.

Nature’s most impressive light shows, the aurora borealis (northern lights) and the aurora australis (southern lights), occur in the ionosphere. Auroras are caused by energized gas molecules in the ionosphere. The molecules are energized by charged particles (called the solar wind) from the Sun, creating beautiful displays of light. The colors of the aurora depend on the types of gases that are energized by the solar wind and the altitude at which they are energized. Auroras are found mostly near the poles because that is where Earth’s magnetic field concentrates the solar wind’s particles.

Aurora displays coincide with solar flare activity on the Sun. Although auroras are occasionally seen at latitudes as low as the southern United States, the best place to see them is at high latitudes during the dark winter months (Figure 1.9).

45