Mesosphere

The Earth is surrounded by the atmosphere, a blanket of gases that has no definite outer edge. Over 80 percent of the gases in the atmosphere are held by gravity within 12.4 mi. (20 km) of the Earth’s surface. The physical and chemical structure of the atmosphere and the interactions occurring there, make the atmosphere an integral part of the Earth system It is held near the surface of the planet by the Earth’s gravitational attraction.

Without the atmosphere, there would be no life on Earth. The atmosphere contains the air people breathe, protects life from harmful radiation from the sun, and helps keep the planet’s heat from the sun from escaping back into space. The atmosphere is made up of a mixture of gases, especially nitrogen, oxygen, argon, and carbon dioxide. It reaches over 310 mi. (500 km) above the surface of the planet. There is no exact boundary between the atmosphere and outer space. Atmospheric gases, however, become thinner higher up in space.

Two Layers of the Atmosphere

Based on chemical composition, the atmosphere is divided into two broad layers: the homosphere and the heterosphere. The homosphere extends up to the height of 56 mi. (90 km) and is characterised by uniformity in chemical composition. It consists of three thermal layers: the troposphere, stratosphere, and mesosphere. Each sub-layer is separated from the adjoining layer by a shallow transitional zone. The heterosphere has heterogeneous chemical composition, with a layered structure of nitrogen, oxygen, helium, and hydrogen.

Generally, the atmosphere consists of five layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The thickness of these layers is slightly different around the globe, and also varies with respect to temperature and season. These five layers of the atmosphere act as safety blankets, regulating the Earth’s temperature and maintaining life forms in it. The mesosphere is the third highest layer and is separated from the stratosphere by the stratopause and from the thermosphere by the mesopause. The top layer of the mesosphere is called the mesopause. The regions of the stratosphere and the mesosphere, along with the stratopause and mesopause, are called the middle atmosphere. The mesosphere is the layer of the Earth’s atmosphere that is immediately above the stratosphere and below the thermosphere, and is located from about 31 mi. (50 km) to 50–56 mi. (80–90 km) in altitude above the Earth’s surface.

Within this layer, temperature decreases with increasing altitude. The mesosphere spreads above the stratosphere from 31 to 50 mi. (50 to 80 km). Temperatures in the upper mesosphere fall as low as minus 148 degrees F (minus 100 degrees C), varying according to latitude and season. Millions of meteors burn up daily in the mesosphere as a result of collisions with the gas particles contained there, creating enough heat to vaporize the falling objects before they reach the ground, resulting in a high concentration of iron and other metal atoms. The meteors, which fall to the Earth with gravitational force, burn because of the friction with the wind at this level. This layer protects the Earth from falling meteors. It is characterized by a broad temperature maximum (near 32 degrees F, 0 degrees C) at its base, from which the temperature decreases to a minimum of about minus 130 degrees F (minus 90 degrees C) at the mesopause level. Mesosphere displays high wispy clouds in high latitudes during the summer because of reflected sunlight from meteriotic dust particles.

The mesosphere is the coldest layer of the atmosphere, where the temperature drops rapidly with altitude. Temperatures are warmest at the lowest level of the mesosphere and become colder at its highest level. The mesosphere is that region of the atmosphere where the air masses are relatively mixed together and the temperature is constant with altitude. The region receives balanced heating from below and above, and cools off about as fast as it warms up. Elevated temperatures can sometimes cause a molecule to become ionized; therefore, the ionosphere and thermosphere can overlap.

The main dynamic features in this region are atmospheric tides, internal atmospheric gravity waves, and planetary waves. In the mesosphere, gravity-wave amplitudes can become so large that the waves’ dissipation transmits so much energy that it largely drives the global circulation of the mesosphere. Because it lies between the maximum altitude for aircraft and the minimum altitude for orbital spacecraft, this region of the atmosphere has only been accessed through the use of sounding rockets. As a result, it is the least understood part of the atmosphere.

Atmospheric Layers

The atmosphere is classified into five different layers,These layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each layer differs in composition and temperature profile.

• Troposphere The layer closest to Earth’s surface, the troposphere, contains most of the mass of the atmosphere. Weather occurs in the troposphere. In the troposphere, air temperature decreases as altitude increases. The altitude at which the temperature stops decreasing is called the tropopause. The height of the tropopause varies from about 16 km above Earth’s surface in the tropics to about 9 km above it at the poles. Temperatures at the tropopause can be as low as –60°C.

• Stratosphere Above the tropopause is the stratosphere, a layer in which the air temperature mainly increases with altitude and contains the ozone layer. In the lower stratosphere below the ozone layer, the temperature stays constant with altitude. However, starting at the bottom of the ozone layer, the temperature in the stratosphere increases as altitude increases. This heating is caused by ozone molecules, which absorb ultraviolet radiation from the Sun. At the stratopause, air temperature stops increasing with altitude. The stratopause is about 48 km above Earth’s surface. About 99.9 percent of the mass of Earth’s atmosphere is below the stratopause.

• Mesosphere Above the stratopause is the mesosphere, which is about 50 km to 100 km above Earth’s surface. In the mesosphere, air temperature decreases with altitude. This temperature decrease occurs because very little solar radiation is absorbed in this layer. The top of the mesosphere, where temperatures stop decreasing with altitude, is called the mesopause.

• Thermosphere The thermosphere is the layer between about 100 km and 500 km above Earth’s surface. In this layer, the extremely low density of air causes the temperature to rise. Temperatures in this layer can be more than 1000°C. The ionosphere, which is made of electrically charged particles, is part of the thermosphere.

• Exosphere The exosphere is the outermost layer of Earth’s atmosphere. The exosphere extends from about 500 km to more than 10,000 km above Earth’s surface. There is no clear boundary at the top of the exosphere. Instead, the exosphere can be thought of as the transitional region between Earth’s atmosphere and outer space. The number of atoms and molecules in the exosphere becomes very small as altitude increases.

In the exosphere, atoms and molecules are so far apart that they rarely collide with each other. In this layer, some atoms and molecules are moving fast enough that they are able to escape into outer space.