Most people don't spend too much time thinking about the air they breathe. They only think about it on days when the news says the air quality is bad, or if they, or someone they know, have asthma or another breathing-related problem. But we should think about the air we breathe and wonder about it. Our atmosphere is a delicate balance of gases that is just right for the life on this planet.
The Earth's atmosphere isn't one thing; it is many layers, and each layer has an important role in making the whole system work. We don't see the atmosphere, unless it is particularly smoggy, but we know it is all around us. In addition to allowing us to breathe, the atmosphere protects us from ultraviolet radiation and keeps the temperatures on the planet from becoming too extreme. The atmosphere is kept intact by Earth's gravity, which may seem odd for something that doesn't have any visible weight. But the gases that make up the atmosphere do contain weight at a molecular level, and so gravity does affect them as well. The bulk of the atmosphere is made up of nitrogen and oxygen, but there are some other gases in the mix as well. There are five layers to the Earth's atmosphere, the exosphere, which is the furthest away, then the thermosphere, mesosphere, stratosphere, and troposphere, which is the area that we live in.
The ionosphere is not a layer of the atmosphere by itself; instead, it consists of all the layers of the upper atmosphere, primarily the thermosphere, but also parts of the exosphere and mesosphere. It begins about 85 kilometers above the surface of the earth and ends at about 600 kilometers. It is this area of the atmosphere that makes radio communication possible, as it reflects radio waves back to Earth, and they are not lost in outer space.
The exosphere is the outermost layer of the atmosphere. Even spatial bodies without atmospheres, like Earth's moon, have exospheres. Earth's exosphere is made up of mostly hydrogen, with some helium, carbon dioxide, and oxygen. The air here is very thin, and it isn't that different from the airlessness of outer space. The outer boundary of the exosphere is believed to be about 120,000 miles above the surface of the earth-about halfway to the moon.
The boundary between the exosphere and the thermosphere is known as the exobase or thermopause. This region of the atmosphere begins about 90 kilometers above the surface of the Earth. How high it goes varies depending on the activity of the sun, but it is generally between 500-1,000 kilometers. Most of the sun's ultraviolet radiation is absorbed by this layer of the atmosphere. Since many scientists say that outer space begins about 100 kilometers above the surface of the Earth, the thermosphere, like the exosphere, is often considered part of space. Temperatures in this region of the atmosphere can get very high, ranging between 500 and 2,000 degrees Celsius-500 degrees Celsius is over 900 degrees Fahrenheit!
The boundary between the thermosphere and the mesosphere is known as the mesopause. The mesosphere begins about 50 kilometers above the Earth's surface and ends at about 85 kilometers. Unlike the thermosphere, the temperature in this region is very cold, typically around -90 degrees Celsius. When meteors enter our atmosphere from outer space, this is the layer where they most often burn up. At the North and South Poles, strange clouds, known as noctilucent clouds, form in the mesosphere. These clouds don't form in any other layer of the atmosphere.
The stratosphere is below the mesosphere, and the boundary between the two is known as the stratopause. The top of the stratosphere is about 50 kilometers above the surface of the Earth. Where it begins varies with the weather and location on the planet. For example, at the equator, the stratosphere begins at about 16 kilometers, but at the poles it begins at about 8 kilometers. There is very little water vapor in the stratosphere; as a result, there are also few clouds. Because the air in this level of the atmosphere is also quite thin, airplanes and weather balloons reach their maximum altitudes here. The temperature in this region varies-it is warmer at the top than at the bottom. At the top of the stratosphere, the temperature averages just below the freezing point of water.
The ozone layer is part of the stratosphere. It protects us from the ultraviolet radiation that makes it through the troposphere, acting like a shield. In the 1980s, scientists began to realize that this layer was being depleted and destroyed. While natural events, like volcanic eruptions, are part of the reason for the ozone layer's depletion, it has been hastened, many argue, by the pollutants people put into the atmosphere. A growing awareness of the chemicals people put into the atmosphere and changes to how we live, for example, regulation car emissions, has improved air quality and slowed the destruction of the ozone layer.
The troposphere is the layer of the atmosphere in which we live. It begins at the surface of the Earth and extends to between 7-20 kilometers above the surface. Just about all weather occurs in this region, and the temperature gets colder toward the upper edge. The area which connects the troposphere and the stratosphere is known as the tropopause. The air also gets thinner as one gets higher in the troposphere, which is noticeable when climbing mountains or visiting a high-altitude city like Denver, Colorado. Unlike the stratosphere, the troposphere contains water vapor, which results in weather events. "Tropos" comes from the Greek word for "change," which is quite appropriate when thinking about the weather fluctuations we often experience. Large thunder clouds often have flat tops because they have reached the tropopause and can travel no higher.
Planetary Boundary Layer:
The planetary boundary layer is the lowest level of the troposphere, and it is also referred to as the atmospheric boundary layer or, simply, the boundary layer. It is altered by its contact with the Earth's surface. Our planet has many different surfaces, for example, mountains, oceans, trees, and manmade structures, and the boundary layer reacts differently with each of them. It tends to be thinner over smooth surfaces like water and thicker over trees, hills, and buildings. Like the troposphere, the boundary layer tends to vary in thickness. While it averages between 200-500 meters, it can be as thin as 50 meters and as thick as 2 kilometers. Because of its closeness to the ground, this is the portion of the atmosphere that we interact with every day. Given how small it is when compared to all the other layers, it is important to keep it clean and pollution-free. A lot of people depend on it!
- Because there is no apparent boundary between outer space and the exosphere, some scientists consider it part of space and not part of our atmosphere.
- The atmosphere Earth has now is probably not the same as the one the planet originally had. Most notably, the original atmosphere probably didn't contain oxygen.
- The International Space Station orbits within the thermosphere.
- The Northern and Southern Lights occur in the thermosphere as well.
- The mesosphere is the hardest area of the atmosphere to study, because weather balloons and planes can't get into that region, and space craft travel above it.
- Strange lightning, known as sprites or elves, often occurs in the mesosphere.
- Felix Baumgartner did his space jump from the stratosphere.
- The ozone layer was actually made by the same ultraviolet radiation it protects us from.
- While ozone in the stratosphere is good, when it occurs in the troposphere, it is unhealthy.
- The Jet Stream travels just below the tropopause, the boundary between the stratosphere and the troposphere.
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