Course: BIS 105 Environmental Biology

Unit 12 Learning Module

Atmospheric Science (2/7)

The atmosphere is layered, and these layers correspond with how the atmosphere's temperature changes with altitude. By understanding the way temperature changes with altitude, we can learn a lot about how the atmosphere works. For example, the reason that weather takes place in the lowest layer is that the Earth's surface is the atmosphere's primary heat source. Heating the lowest part of the atmosphere places warm air beneath colder air, an unstable situation that can produce violent weather. Interesting things happen higher in the atmosphere, like the beautiful aurora, which light up the sky with brilliant flashes, streaks and rolls of white or colored light.

Air Temperature

Warm air rises: that's a saying just about everyone has heard. But maybe not everyone knows why this is true. Gas molecules are free to move around, and the molecules can take up as much space as they need. When the molecules are cool, they are sluggish and do not move as much, so they do not take up as much space. When the molecules are warm, they move vigorously and take up more space. With the same number of molecules in this larger volume, the air is less dense and air pressure is lower. This warmer, lighter air is more buoyant than the cooler air above it, so it rises. The cooler air then sinks down, since it is more dense than the air beneath it. The rising of warmer air and sinking of cooler air is a very important concept for understanding the atmosphere.

As you learned in the previous section, the composition of gases is mostly the same throughout the first 100 km of our atmosphere. This means if we measure the percentages of different gases throughout the atmosphere, it will stay basically the same. However the density of the gases and the air pressure do change with altitude; they basically decrease with increasing altitude. The property that changes most strikingly with altitude is air temperature. Unlike the change in pressure and density, changes in air temperature are not regular. A change in temperature with distance is called a temperature gradient.

The atmosphere is divided into layers based on how the temperature in that layer changes with altitude, the layer's temperature gradient. The temperature gradient of each layer is different. In some layers, temperature increases with altitude and in others it decreases. The temperature gradient in each layer is determined by the heat source of the layer. The different temperature gradients in each of the four main layers create the thermal structure of the atmosphere.

There are several layers of the atmosphere. The first layer is the troposphere. It is the closest to the ground and is sometimes referred to as the lower atmosphere. The second layer is the stratosphere, and is sometimes referred to as the upper atmosphere. Most of the important processes of the atmosphere take place in one of these two layers.

Troposphere

About three-fourths of the gases of the atmosphere are found in the troposphere because gravity pulls most of the gases close to the Earth's surface. As with the rest of the atmosphere, 99% of the gases are nitrogen and oxygen.

The thickness of the troposphere varies around the planet. Near the equator, the troposphere is thicker than at the poles, since the spinning of the Earth tends to shift air towards the equator. The thickness of the troposphere also varies with season. The troposphere is thicker in the summer and thinner in the winter all around the planet. At the poles in winter, the atmosphere is uniformly very cold and the troposphere cannot be distinguished from other layers. The importance of this feature of the atmosphere will become clear when we learn about ozone depletion.

Earth's surface is a major source of heat for the troposphere. Where does the heat come from? Nearly all the heat comes from the sun, either directly or indirectly. Some incoming sunlight warms the gases in the atmosphere directly. But more sunlight strikes the Earth's rock, soil, and water, which radiate it back into the atmosphere as heat, further warming the troposphere. The temperature of the troposphere is highest near the surface of the Earth and declines with altitude. On average, the temperature gradient of the troposphere is 6.5°C per 1,000 m (3.6°F per 1,000 feet) of altitude.

Notice that in the troposphere, warm air is beneath cold air. Since warm air is less dense and tries to rise, this condition is unstable. So the warm air at the base of the troposphere rises and cool air higher in the troposphere sinks. For this reason, air in the troposphere does a lot of mixing. This mixing causes the temperature gradient to vary with time and place. For reasons that will be discussed in the next section, rising air cannot rise above the top of the troposphere. The rising and sinking of air in the troposphere means that all of the planet's weather takes place in the troposphere.

When there is a temperature inversion, air temperature in the troposphere increases with altitude and warm air sits over cold air. This is called an inversion because the usual situation is reversed or inverted. Inversions are very stable and they often last for several days or even weeks. Inversions commonly form over land at night or in winter. At these times, the ground is cold because there is little solar energy reaching it. At night, the Sun isn't out and in winter, the Sun is at a low angle, so little solar radiation reaches the ground. This cold ground cools the air that sits above it, making this low layer of air denser than the air above it. An inversion also forms on the coast where cold seawater cools the air above it. When that denser air moves inland, it slides beneath the warmer air over the land. Since temperature inversions are stable, they often trap pollutants and produce unhealthy air conditions in cities.

Smoke makes a temperature inversion visible. The smoke is trapped in cold dense air that lies beneath a cap of warmer air.

At the top of the troposphere is a thin layer called the tropopause. Temperature in the tropopause does not change with height. This means that the cooler, denser air of the troposphere is trapped beneath the warmer, less dense air of the stratosphere. So the tropopause is a barrier that keeps air from moving from the troposphere to the stratosphere. Sometimes breaks are found in the tropopause and air from the troposphere and stratosphere can mix.

Stratosphere

The stratosphere rises above the tropopause. When a volcano erupts dust and gas that makes its way into the stratosphere, it remains suspended there for many years. This is because there is so little mixing between the stratosphere and troposphere. Pilots like to fly in the lower portions of the stratosphere because there is little air turbulence.

In the stratosphere, temperature increases with altitude. The reason is that the direct heat source for the stratosphere is the Sun. A layer of ozone molecules absorbs solar radiation, which heats the stratosphere. Unlike in the troposphere, air in the stratosphere is stable because warmer, less dense air sits over cooler, denser air. As a result, there is little mixing of air within the layer.

The stratosphere has the same composition of gases as the rest of the atmosphere, with the exception of the ozone layer. The ozone layer is found within the stratosphere at between 15 to 30 km (9 to 19 miles) altitude. The thickness of the ozone layer varies by the season and also by the latitude. The amount of ozone present in the ozone layer is tiny, only a few molecules per million air molecules. Still, the concentration of ozone is much greater than in the rest of the atmosphere. The ozone layer is extremely important because ozone gas in the stratosphere absorbs most of the Sun’s harmful ultraviolet (UV) radiation.

How does ozone do this? High energy ultraviolet light, traveling through the ozone layer, breaks apart the ozone molecule, O_3 into one oxygen molecule (O_2) and one oxygen atom (O). This process absorbs the Sun's most harmful UV rays. Ozone is also reformed in the ozone layer: oxygen atoms bond with O_2 molecules to make O_3. Under natural circumstances, the same amount of ozone is continually being created and destroyed and so the amount of ozone in the ozone layer remains the same.

The ozone layer is so effective that the highest energy ultraviolet, the UVC, does not reach the planet's surface at all. Some of the second highest energy ultraviolet, UVB, is stopped as well. The lowest energy ultraviolet, UVA, travels through the atmosphere to the ground. In this way, the ozone layer protects life on Earth. High energy ultraviolet light penetrates cells and damages DNA, leading to cell death (which we know as a bad sunburn). Organisms on Earth are not adapted to heavy UV exposure, which kills or damages them. Without the ozone layer to reflect UVC and UVB, most complex life on Earth would not survive long.

Above the stratosphere is the thin stratopause, which is the boundary between the stratosphere below and the mesosphere above. The stratopause is at about 50 km above the Earth's surface.

Mesosphere

Temperatures in the mesosphere decrease with altitude. Since there are very few gas molecules in the mesosphere to absorb the Sun's radiation, the heat source here is the stratosphere below. The mesosphere is extremely cold, especially at its top, about -90°C (-130°F).

The air in the mesosphere is extremely thin: 99.9% of the mass of the atmosphere is below the mesosphere. As a result, air pressure is very low. Although the amount of oxygen relative to other gases is the same as at sea level, there is very little gas and so very little oxygen. A person traveling through the mesosphere would experience severe burns from ultraviolet light since the ozone layer which provides UV protection is in the stratosphere below them. And there would be almost no oxygen for breathing! Stranger yet, an unprotected traveler's blood would boil at normal body temperature because the pressure is so low.

Despite the thin air, the mesosphere has enough gas that meteors burn as they enter the atmosphere. The gas causes friction with the descending meteor, producing its tail. Some people call them "shooting stars". Above the mesosphere is the mesopause. Astronauts are the only people who travel through the mesopause.

Meteors burn up as they hit the mesosphere.

Thermosphere and Beyond

The thermosphere rises from the mesopause. The International Space Station (ISS) orbits within the upper part of the thermosphere, at about 320 to 380 km above the Earth.

The International Space Station.

What does an astronaut experience in the thermosphere? Temperatures in the thermosphere can exceed 1000°C (1800°F) because oxygen molecules in the layer absorb short wavelength solar energy. Yet despite these high temperatures, the atmosphere outside the ISS feels cold. This is because gas molecules are so few and far between that they very rarely collide with other atoms and so little energy is transferred. The density of molecules is so low that one gas molecule can go about 1 km before it collides with another molecule.

Within the thermosphere is the ionosphere. The ionosphere gets its name because nitrogen and oxygen molecules are ionized by solar radiation. In the process of ionization, the neutrally-charged molecules absorb high-energy, short-wavelength energy from the Sun. This causes the molecules to lose one or more electrons and become positively-charged ions. The freed electrons travel within the ionosphere as electric currents. Because of the free ions, the ionosphere has many interesting characteristics.

Have you ever been out on an open road and found a radio station on the AM dial that is transmitted from hundreds of kilometers away? The reason radio waves can travel so far at night involves the ionosphere. During the day, the lower part of the ionosphere absorbs some of the energy from the radio waves and reflects some back to Earth. But at night the waves bounce off of the ionosphere, go back down to the ground, and then bounce back up again. This does not happen during the day due to ionization in the ionosphere. This bouncing up and down allows radio waves to travel long distances.

The most spectacular feature of the ionosphere is the nighttime aurora. Spectacular light displays with streamers, arcs, or foglike glows are visible on many nights in the polar regions. The lights can be white, green, blue, red or purple. The display is called the aurora borealis or northern lights in the Northern Hemisphere. It is called the aurora australis or southern lights in the Southern Hemisphere.

The Northern Lights above Bear Lake, Alaska.

The aurora is caused by massive storms on the Sun that release great quantities of protons and electrons. These electrically charged particles fly through space and spiral in along lines of Earth's magnetic field. Earth's magnetic field guides the charged particles toward the poles, which explains why the auroras are seen primarily in the polar regions. When the protons and electrons enter the ionosphere, they energize oxygen and nitrogen gas molecules and cause them to light up. Each gas emits a particular color of light. Depending on where they are in the atmosphere, oxygen shines green or red and nitrogen shines red or blue. The frequency and intensity of the aurora increases when the Sun has more magnetic storms.

The outermost layer of the atmosphere is the exosphere. There is no real outer limit to the exosphere. If you continued traveling farther out from the Earth, the gas molecules would finally become so scarce that you would be in outer space. There is so little gravity holding gas molecules in the exosphere that they sometimes escape into outer space. Beyond the atmosphere is the solar wind. The solar wind is made of high-speed particles, mostly protons and electrons, traveling rapidly outward from the Sun.