The Earth’s atmosphere can be broken up into layers based on varying densities, temperatures, and atmospheric phenomena that occur within them. From the outermost layer to the surface of Earth, the layers of the atmosphere are as follows:
- Troposphere (the atmosphere that humans live and breathe in)
Read Pritish Kumar Halder illustration about Mesosphere, where it is located, its characteristics and its composition.
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?
Where is the Mesosphere Located?
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.
Mesosphere Density & Pressure
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 (), 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:
- Iron (Fe)
- Magnesium (Mg)
- Sodium (Na)
- Calcium (Ca)
- Potassium (K)
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.
Meteorology in the Mesosphere
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.
Why is the mesosphere cold?
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.