Air In Our Atmosphere
The Earth’s atmosphere is an envelope surrounding the planet that makes it possible for life to exist and is broken up into five major spheres: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. Learn more about the air in our atmosphere with our free guide below.
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The Earth’s atmosphere is an envelope surrounding the planet that makes it possible for life to exist. It acts as a mostly invisible natural force field that not only keeps the air we all breathe from escaping into space, but also prevents dangerous cosmic rays such as solar radiation from saturating the Earth and making it uninhabitable. Earth’s atmosphere is composed of multiple layers of protective barriers, starting with the troposphere and ending with the exosphere, whose upper limit is roughly 6,200 miles above the surface of the Earth. These layers are responsible for both trapping heat and reducing heat, effectively preventing Earth from freezing like Mars, or overheating like Venus. The atmosphere becomes thicker and heavier near the ground and thinner as it approaches outer space.
The word atmosphere is a combination word of the Greek word atmos, which means air or vapor in English, and sphaera, which is Latin for sphere or globe. The Earth’s atmosphere is broken up into five major spheres. The lowest layer is the troposphere. The word troposphere comes from tropos, which in Greek means turning or circulation. The word stratosphere comes from the Latin word strato, which means layer in English. Mesosphere comes from Mesos, a Greek word for middle, while thermosphere stems from the Greek word thermo, which means heat. Finally, the exosphere is named after the Greek word exo, or outside in English.
The troposphere is the part of the atmosphere that is most familiar. Because it is the layer that starts from sea level and extends up to between 6 and 9 miles above sea level, it is the lowest of all layers, and where almost all life on Earth lives. Most of the air we breathe, a mix of oxygen and nitrogen along with other gases, exists in the troposphere. The troposphere received its name because it is where almost all weather occurs. The tropopause is where the jet stream, a fast-moving current of air that primarily moves from west to east, or a westerly direction, around the world. Typically, the temperature of the air cools as it rises higher into the troposphere, potentially reaching a low of between 65 and 100 degrees below zero. The world’s highest mountain, Mount Everest, reaches near the top of this atmospheric layer, which is called the tropopause.
This part of the atmosphere starts at an altitude of up to nine miles above sea level and reaches as high as 30 miles. Unlike in the troposphere, the air in this region gets warmer with higher altitudes, because of the amount of ozone, a gas that heats up when it absorbs ultraviolet radiation. The stratosphere also contains the ozone layer, which is important to life on Earth because it prevents ultraviolet radiation from reaching the surface and harming living things. Jet propelled aircraft prefer to fly at the lowest reaches of the stratosphere or in the upper reaches of the troposphere because most storms occur below these altitudes. In addition, the combination of cooler air and lower air drag provide jet engines better fuel efficiency. Thunderstorms and some powerful hurricanes, however, produce clouds that reach into the stratosphere and cause turbulence for aircraft. Generally, the stratosphere is extremely dry, which means that nacreous clouds form in this region. Nacreous clouds are thin wisps of clouds, consisting entirely of ice crystals that form in the stratosphere.
The stratopause layer is the highest part of the stratosphere, and it lies on the boundary between the stratosphere and the mesosphere. The warmest temperatures in the stratosphere reach as high as 32 degrees Fahrenheit, which is the point where water freezes. Some bacteria have been known to survive in this region, and some birds have run into airplanes at the lower altitudes of the stratosphere. As a result, the stratosphere, along with the troposphere, is considered to be a part of Earth’s biosphere.
At roughly 31 miles above the surface, the mesosphere begins. This layer reaches as high as 50 to 62 miles. The mesosphere is characterized by the resumption of the inverse relationship between height and temperature. This means that, like the troposphere, the air gets colder as the altitude increases. Temperatures in the mesosphere reach as low as 130 degrees below zero, especially in the mesopause zone. Also, it is in the mesosphere that meteors are first seen burning up in the atmosphere. The mesopause lies at the border of the mesosphere and the thermosphere.
The thermosphere, starting at an altitude between 50 and 62 miles, is characterized by the fact that temperatures rise to over 2,500 degrees Fahrenheit in this zone. That’s hot enough to melt steel. The air is so thin, however, that thermometer readings in the thermosphere will read freezing temperatures. This is because there is almost no molecular friction in this layer. While the atmosphere officially ends with the exosphere, to become recognized as an astronaut one only needs to reach the Karman Line. The Karman Line is at an altitude of roughly 62 miles, which is in the thermosphere. The International Space Station also maintains a stable orbit above the Earth in the middle area of the mesosphere, at an altitude of over 200 miles. The thermopause is the boundary of this layer, which leads to the exosphere.
The exosphere is the edge of the Earth’s atmosphere, and it begins at an altitude of at least 370 miles. While temperatures in the exosphere reach over 4,500 degrees Fahrenheit, which is hot enough to melt almost any known metal, a thermometer will freeze instantly in this environment. Certain satellites travel in the exosphere in their orbit around Earth, because of the lack of atmospheric drag. The atmosphere is at its thinnest in this zone, and the exosphere often referred to as outer space. It extends to a minimum altitude of 6,200 miles, but other estimates place its boundary at half the distance between the Earth and the moon.
In addition to the five major layers of the atmosphere, there are a number of minor layers that exist. For instance, the planetary boundary layer is a variable zone within the troposphere, although it rarely exceeds 0.6 miles in altitude from sea level. Its size changes with the temperature of the day, and tends to be thicker during warm temperatures. This is because warmer air rises, thus increasing the altitude of the boundary of the layer. Its thickness can be determined by changes in temperature and wind speeds.
The ionosphere is a zone that begins at an altitude of 43 miles, which is within the mesosphere, and encompasses the thermosphere up to over 350 miles above sea level, near the thermopause zone and the exosphere. This layer is characterized by the interaction between electrically charged atoms and ultraviolet radiation from the sun. This interaction is what causes the phenomenon known as the Aurora Borealis, or northern lights, which occur in the thermosphere portion of the ionosphere. It is also an important area for HAM radio operators, who use the ionosphere to bounce signals to recipients. Solar flares interact with this zone to disrupt communications, and fluctuations in the ionosphere can also reduce the accuracy of Global Positioning Satellite (GPS) data.
The thermosphere is host to the boundary between two more minor layers, called the homosphere and the heterosphere. Up to an altitude of 62 miles above sea level, the atmosphere is made of a uniform mixture of chemicals at all altitudes, and this zone is called the homosphere. Above this line, the composition of the atmosphere is less uniform, varying at different altitudes, and it is called the heterosphere. The line where the heterosphere and homosphere meet is called the turbopause. The turbopause area lies between the mesosphere and thermosphere.
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- Windows to the Universe: The Troposphere
- WeatherOnline: Troposphere
- The National Center for Atmospheric Research: The Layered Atmosphere
- Pittsburg Unified School District: Earth’s Atmosphere
- University of Texas, Dallas: The Layers of Earth’s Atmosphere (PDF)
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- Mesosphere & Mesopause
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- Swarthmore College: Mesosphere
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- High Frequency Active Auroral Research Program: About the Ionosphere
- University of Maryland Baltimore: The Earth and Its Atmosphere
- MIT: Thermosphere
- Miami Museum of Science: Thermosphere
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- The Encyclopedia of Science: Exosphere
- The Planetary Boundary Layer
- National Weather Service: Ionosphere
- Geography For Kids, The Study of Our Earth: The Layers of Our Atmosphere
- Rice University: The Atmosphere
- The Geological Society of America: Layers of the Atmosphere (PDF)
- Mooresville Graded School District: Layers of the Atmosphere