- Introduction
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The Basics
The earth's atmosphere is vital to the existence of life on earth.
It provides the air that we breathe.
The atmosphere is also responsible for creating a planet that has temperatures that can support life.
The earth's atmosphere plays a major role in creating the earth's weather. The atmosphere is where most weather events are generated and occur. Think back to Unit 3 and the water cycle. The atmosphere is an integral part of that cycle. This is important to earth for supplying water to earth's surface in the form of precipitation.
Composition
The earth's atmosphere is a layer of gases that extend from earth's surface to space. The atmosphere envelops completely surrounds the earth like a blanket. The earth's atmosphere is composed of
- 78% nitrogen (N2),
- 21% oxygen (O2), and
- 1% water vapor and other trace gases, such as carbon dioxide, carbon monoxide, methane, and argon.
All of these gases combine to absorb ultraviolet radiation from the sun. This warms earth's surface through heat retention.
This pie chart illustrates the breakdown by percentage of gases in the Earth's atmosphere. Note that more than 3/4 of the earth's atmosphere is nitrogen and most of the rest is oxygen. Only a very small sliver - around 1% - is carbon dioxide, argon, and other trace gases.
The earth's atmosphere is divided into 4 different layers — troposphere, stratosphere, mesosphere, and thermosphere — based on physical and chemical characteristics of the atmosphere.
Troposphere
The troposphere is the layer that touches the earth's surface at sea level and extends up from 4-12 miles. Most of the mass in the earth's atmosphere, 75-80%, is located in the troposphere. The troposphere is warmed by the earth's surface which receives heat from the sun's radiation. Convection currents
created by warm air rising and cooler air sinking
are responsible for heating throughout the troposphere. |
Stratosphere
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The stratosphere is the atmospheric layer directly above the troposphere. This layer extends from about 8 to 30 miles above sea level. The mass of the stratosphere makes up about 15% of the earth's atmosphere. No weather takes place in the stratosphere. It is cloud-free, except for occasionally when large anvil clouds called thunderheads reach the bottom layer of the stratosphere. Most airplanes cannot fly in the stratosphere because the air is not dense enough to provide enough lift to keep them airborne. This layer is where ozone (O3) is located. Ozone is the molecule that absorbs harmful ultraviolet radiation from the sun, protecting organisms on earth from its harmful effects. This makes the temperature distribution different from the troposphere. In the stratosphere, as altitude increases, temperature slowly increases. |
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Mesosphere
 The mesosphere is the atmospheric layer above the stratosphere. Approximately 99.9% of the mass in the atmosphere is located below the mesosphere. The mesosphere starts about 31 miles above sea level and extends to about 53 miles. There is no ozone in this layer, so solar radiation passes through without being absorbed. Therefore as you increase in altitude in the mesosphere, the temperature decreases. The lower altitudes of the mesosphere get some radiant energy from the stratosphere through convection. The top of the mesosphere is the coldest part of the atmosphere. The mesosphere is hard to study because scientific instruments, such as weather balloons, and aircraft cannot fly in this layer. This is where most meteors
small pieces of rock, dust, or metal from space that produce streaks of light as they burn: sometimes called falling or shooting stars.
from space burn up. |
Thermosphere
The thermosphere is the layer of the earth's atmosphere that is located between the mesosphere and exosphere has very little air and is very similar to the vacuum of space . The temperatures in the thermosphere are the greatest because oxygen molecules absorb energy from the sun. Temperatures within the thermosphere are typically around 200 degrees Celsius and can reach temperatures of better than 1000 degrees Celsius. The thermosphere extends from about 56 miles above earth to 311 to 621 miles above earth. The air density is so low in the thermosphere that it is often considered outer space. The space shuttle and International Space Station both orbit the earth in the thermosphere. There are not distinct boundaries where one layer of the atmosphere stops and the next begins. Much like with biomes that we discussed in Unit 1, there are transition areas where you have characteristics of both layers.
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Summary
Visit Layers of Earth's Atmosphere to learn more about the layers of the earth's atmosphere.
This graph illustrates the change in temperature based on altitude for the layers of the atmosphere. Height is measured along the vertical axis in km on the left and in miles on the right. Temperature is along the horizontal axis. Please note that temperature decreases as you move further up in the troposphere, stabilizing at -40°C, then increases back up to 0°C throughout the stratosphere. Temperature decreases again in the mesosphere down to almost -100°C before rising and exceeding 60°C through the thermosphere.
Click for a larger version.
Notice that temperature is both directly proportional and inversely proportional to change in altitude depending on the layer.
Also take note that atmosphere pressure is inversely proportional to the increase in altitude — as the altitude increases, atmospheric pressure decreases.
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