Earth's Atmosphere

Lesson Plan - Get It!

Audio:

• What is air?

There is a thin layer of air surrounding the earth. This air is our atmosphere. Air is not specific — it's the mixture of gases surrounding the earth.

Most gas is nitrogen (78%) or oxygen (21%). The other 1% is a mixture of argon (0.9%), carbon dioxide (0.03%), and trace amounts of neon, helium, methane, water vapor, krypton, hydrogen, and xenon.

The air may contain pollen depending on your location and time of year.

Air is not evenly distributed throughout the atmosphere. If you were to travel up from the earth's surface, there would be less and less air the higher you go.

• Why?

Gravity!

Gravity pulls the air molecules toward the earth, meaning that the thickest air is above the earth's surface. As you move upward and your altitude increases, the air becomes increasingly thinner.

There are fewer air molecules at high elevations, including oxygen molecules. This makes it harder to breathe at high altitudes.

Extreme mountain climbers bring oxygen tanks when the air becomes too thin, and they can no longer get enough oxygen through normal breathing.

To learn more about air pressure, check out the Elephango lesson found under Additional Resources in the right-hand sidebar.

Our atmosphere is separated into distinct layers. Sometimes, the temperature increases as you move upward; sometimes, it decreases. Every time the temperature change reverses direction, it marks the boundary between separate atmospheric layers.

Let's start at the surface and look at each layer, one at a time. You'll be learning a lot of information, so you should have a way to organize your notes. Print the Atmospheric Layers Note Sheet found under Downloadable Resources in the right-hand sidebar, and fill it in as you read.

Layer 1: Troposphere

The troposphere starts at ground level and extends to about 10 km (approximately 6 miles or 33,000 feet). When moving upward through the troposphere, the temperature steadily decreases. This is why high points, like mountaintops, have snow on them.

The troposphere contains almost all the water vapor in our atmosphere. This means that clouds and weather exist only in the troposphere.

At the top of the troposphere is the tropopause section, the boundary between the troposphere and the next layer. In the tropopause, the temperature stops decreasing at around -60°F (-15°C) and remains fairly steady before increasing as we enter the next layer: the stratosphere.

Layer 2: Stratosphere

The stratosphere begins where the troposphere ends and extends to about 50 km (31 miles) above the earth.

Moving up through the stratosphere, the temperature increases instead of decreases. It doesn't get boiling, though, and the temperature stops increasing right before you get above the freezing point (0°C, 32°F).

Although too high for most commercial airplanes, large jets may fly in the stratosphere to avoid the weather systems found in the troposphere.

The stratosphere is relatively calm because it is already cool on the bottom and warm on top, so there isn't a lot of conventional air movement. Unfortunately, this is also why pollutants released into the atmosphere stay in the almost stagnant stratosphere for long periods.

This is even worse because our ozone layer is located in the stratosphere. The ozone layer is made of — you guessed it — ozone (O₃). Ozone protects us from some of the sun's most harmful rays.

As you reach the top of the stratosphere (the stratopause), the air is now roughly 1,000 times thinner than at the earth's surface.

Layer 3: Mesosphere

The mesosphere exists from about 50 km (31 miles) to about 85 km (53 miles) above the earth. Moving upward through the mesosphere, the temperature once again decreases.

Not much is known about the mesosphere because it is too high for aircraft or even weather balloons to reach. Even satellites can't give us much information because they orbit the earth above the mesosphere and cannot directly measure the conditions within this mysterious layer.

Most meteors that would otherwise hit the earth are vaporized in the mesosphere. Some bits of meteor remain in this layer, which causes this layer to have a slightly higher concentration of metals.

At the top of the mesosphere, you reach the mesopause before entering the fourth layer of our atmosphere: the thermosphere.

Layer 4: Thermosphere

The thermosphere extends from about 90 km (56 miles) to between 500 and 1,000 km (311 and 621 miles) above the earth. Outer space officially begins in this layer at an altitude of 100 km (62 miles). It would only take about an hour of driving at highway speed to reach this point on our journey!

The thermosphere is very hot during the day but cold at night. The air density is so low up here that most of the thermosphere is technically considered a part of space. When the sun is particularly active, the thermosphere puffs up to absorb more X-ray and UV radiation.

Due to the extreme temperatures, it would be difficult to allow humans to spend time in the thermosphere; however, this is where most satellites orbit the earth. The satellites are used for various purposes, including sending global positioning data (GPS), radio and TV signals, and even weather measurements back to the earth.

The thermosphere is also home to the aurora, the earth's northern and southern lights. These lights are produced when charged particles from space collide with molecules and atoms. This sends the particles into a higher energy state, and then that extra energy is emitted as the light we see.

Layer 5: Exosphere

The exosphere is our final atmospheric layer. Here, the air is so thin that it is nearly identical to the conditions in outer space.

The bottom of the exosphere (and top of the thermosphere) is called the thermopause or exobase and is found at roughly 1,000 km (620 miles) above our planet.

There's not a lot happening in this layer. Even the International Space Station (ISS) orbits below the exosphere. Gas particles are spread so far apart that they rarely even collide, instead spending their time floating aimlessly and undisturbed.

The top of the exosphere is difficult to pinpoint because it gradually fades into outer space. Some scientists don't even think the exosphere is a part of our atmosphere and that the thermosphere should be the top layer. However, most scientists do include the exosphere as an atmospheric layer.

• What do you think?

Use the Exosphere T-chart found under Downloadable Resources in the right-hand sidebar to organize your thoughts. On one side of the chart, list reasons why the exosphere should be considered an atmosphere layer. On the other side, list reasons against it.

Once you've completed your Exosphere T-chart, move on to the Got it? section to organize our atmosphere!