The Earth's atmosphere can be broken up into layers based on varying densities, temperatures, and atmospheric phenomenon that occur within them. From the outermost layer to the surface of Earth, the layers of the atmosphere are as follows:
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This lesson will focus entirely on the third and middle layer of the atmosphere: the mesosphere. The word mesosphere comes from the Greek word mesos, which means middle. But what is the mesosphere? Exactly where is the mesosphere located? And what makes it different from the other layers?
The mesosphere is located directly above the stratosphere and directly below the thermosphere. The Earth's atmospheric layers range and fluctuate in height and density due to fluctuations in absorbed solar radiation and therefore the amount of thermal expansion throughout the atmosphere. However, the mesosphere generally begins between 50 and 65 kilometers above Earth's surface and ends approximately 85 kilometers above the surface. The mesosphere is therefore approximately 35 kilometers thick. Additionally, the transitionary zone where the mesosphere ends is called the mesopause which exists at altitudes from 80-90 kilometers above Earth's surface, separating the rest of the mesosphere from the thermosphere.
The mesosphere temperature varies throughout. With an increase in altitude within the mesosphere, the temperature decreases because of a decrease in the absorption of penetrating solar radiation. The temperature in the mesosphere ranges from -2.5 degrees Celsius to -90 degrees Celsius. However, the mesopause can reach temperatures below -143 degrees Celsius. Generally, the atmosphere gets cooler with an increase in altitude because an increase in distance from the Earth's surface, which is a source of thermal radiation that is being reemitted after the absorption of solar radiation. Additionally, with an increase in altitude comes a decreased atmospheric density and therefore a decrease in the amount of solar radiation absorbed and reemitted. The ozone layer within the stratosphere also absorbs a lot of solar radiation and reemits it. So, the temperature does increase with altitude through the stratosphere, but begins to decrease again at the top of the stratopause and throughout the mesosphere above it.
Generally, the entire atmosphere decreases in both density and pressure with an increase in altitude. This is due to the decrease in gravitational attraction of the atmosphere with distance from the Earth's center mass. Additionally, the temperature rapidly increases within the thermosphere, which increases that layer's thermal expansion and therefore decreases the density. The pressure of the mesosphere ranges from 0.122 Newtons per square meter to 0.99 Newtons per square meter. Mesosphere density ranges from 0.102 kilograms per cubic meter to 0.994 kilograms per cubic meter.
Generally, the Earth's atmosphere is composed of 78% nitrogen (N), 21% oxygen (O), 0.93% argon (Ar), 0.04% carbon dioxide ({eq}CO_2 {/eq}), and 0.03% of trace gases. This composition holds true throughout most of the atmosphere as the differentiation by molecular weight or mass does not occur, because the turbulence and circulation of the atmosphere keeps the composition relatively uniform or homogenous. However, meteors begin to burn up and ionize upon entering the mesosphere due to the sudden increase in atmospheric density transitioning from the thermosphere to the mesosphere. Meteors cause some metallic elements to be present within the mesosphere such as the following:
However, below 85 kilometers, the above elements form compounds within the mesosphere including oxides, hydroxides, and carbonates. These compounds polymerize or clump together into meteoric smoke particles (MSPs) nanometers in size. It is likely that MSPs act as condensation nuclei and form cloud condensation around them in the mesosphere. Gradually, MSPs sink and make their way to being deposited onto Earth's surface after approximately 4 years.
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Besides the ionization and burning up of shooting stars or meteors within the Earth's mesosphere, there are other meteorological or atmospheric phenomena that also occur that make the mesosphere distinct from other layers of the atmosphere.
Within the mesosphere there are unique cloud formations called noctilucent clouds or polar mesospheric clouds.
Polar mesospheric clouds (PMCs), also called noctilucent clouds and "night-shining" clouds, are clouds that consist of ice crystals that form within the mesosphere and are the highest cloud formation within the entire atmosphere. These formations are only visible at astronomical twilight, when the Sun is hidden below the horizon. At this position, the noctilucent clouds are so high in the atmosphere that they are sunlit for a period of time while the Sun is below the horizon. The clouds are too thin and faint to be seen during full daylight.
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Strange electrical phenomena also occur within the mesosphere such as sprites and ELVES.
Sprites are massive electric discharge storms that occur within the mesosphere, but high above thunderstorm or cumulonimbus clouds which occur within the troposphere. These massive electrical discharges appear as varied ranges of shapes or red or orange luminous flashes and flickers. Though sprites appear to be a phenomena like lightning, sprites are a cold plasma.
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ELVES is an acronym short for "Emissions of Light and Very Low Frequency Perturbations due to Electromagnetic Pulse Sources." This electrical phenomenon appears as a dim red glow within the mesosphere that can have a diameter of up to 400 kilometers in size.
The mesosphere is also the location of many atmospheric tides. Mainly within the mesosphere are strong zonal winds that run east to west. Additionally, there are atmospheric tides which occur because of the gravitational influence of the moon orbiting around Earth. The gravitational influence of the Sun, moon, and Earth cause tidal bulges to not only occur within Earth's oceans, but also within the atmosphere. Gravity waves are also generated by the displacement of Earth through its orbit around the Sun, and its wobble caused by the moon's orbit. This displacement of solid Earth causes fluid Earth (such as air and water) to be displaced in respect to solid Earth and therefore due to buoyancy, returns to equilibrium and results in waves. These waves propagate from the troposphere, through the stratosphere, and up into the mesosphere, where they dissipate due to the decrease in atmospheric density with altitude.
Studying the mesosphere is difficult. Typically, meteorologists use satellites or weather balloons to study atmospheric phenomenon within layers such as the troposphere and stratosphere. However, the mesosphere is too thin for weather balloons to continue beyond anywhere above 53 kilometers. Additionally, the mesosphere has too much density and atmospheric drag for orbital spacecraft such as satellites to enter. If a satellite descended into the mesosphere, it would experience too much air resistance, experience orbital decay, and crash into the Earth.
Instead of using these devices, meteorologists have to rely on sounding rockets which are suborbital research rockets. These rockets are launched into the mesosphere where they can maintain altitude and flight for a few minutes, during which measurements are taken by equipment attached to the rocket.
The mesosphere is the third, middle layer of Earth's atmosphere. The mesosphere is located approximately between 50 and 85 kilometers above Earth's surface. This region of the atmosphere is too thin for weather balloons to reach, and too thick for satellites to enter; therefore, scientists must use suborbital rockets to study and measure the mesosphere.
The mesosphere is composed of the same gases as the rest of the atmosphere. However, because meteors and "shooting stars" burn up in this layer of the atmosphere, it also contains meteoric smoke particles which contain atoms of metallic elements such as iron, magnesium, calcium, sodium, and potassium. The mesosphere is the coldest layer of the atmosphere and decreases in temperature with altitude. It gets so cold that clouds called noctilucent clouds are composed of ice crystals. The coldest portion of the mesosphere is the mesopause which is the boundary that separates the mesosphere from the thermosphere.
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The mesosphere decreases in temperature with altitude. The mesosphere ranges in temperatures from -2.5 to -90 degrees Celsius. The reason the temperature decreases with altitude is because the Earth's surface is a source of heat; heat from its geothermal gradient, and absorbed solar radiation that is reemitted. An increase in distance from Earth's heat decreases the temperature.
The mesosphere has the same composition as the other layers of Earth's atmosphere. The composition of the atmosphere is generally 78% nitrogen (N), 21% oxygen (O), 0.93% argon (Ar), 0.04% carbon dioxide (CO_2), and 0.03% of trace gases. However, the mesosphere also contains meteoric smoke particles (MSPs) consisting of compounds that contain metallic elements such as the following:
Iron (Fe)
Magnesium (Mg)
Sodium (Na)
Calcium (Ca)
Potassium (K)
The main characteristics of the mesosphere that differentiate it from other layers of Earth's atmosphere are the following:
The mesosphere is the coldest layer of Earth's atmosphere.
The mesosphere is the first line of defense from bodies entering the atmosphere; meteors begin to burn up upon entering the mesosphere.
Noctilucent clouds form high up in the mesosphere and are only visible at twilight.
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