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, namely, the troposphere, the stratosphere, and the mesosphere. Each sub-layer is separated from the adjoining layer by a shallow transitional zone. The het-erosphere has heterogeneous chemical composition, with layered structure, of nitrogen, oxygen, helium, and hydrogen, respectively. Generally, the atmosphere consists of five layers, namely, the troposphere, the stratosphere, the mesosphere, the thermosphere, and the 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. (8090 km.) altitude above the earth's surface. Within this layer, temperature decreases with increasing altitude. The mesosphere spreads above the stratosphere from 31-50 mi. (50-80 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 withgravitational force, burn due to the friction with the wind at this level. This layer protects the Earth from the 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 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.
sEE ALsO: Carbon Dioxide; Evolution of the Atmosphere; Stratosphere; Thermosphere; Weather.
BIBLIOGRAPHY. Guy P. Brasseur and Susan Solomon, Aer-onomy of the Middle Atmosphere: Chemistry and Physics of the Stratosphere and Mesosphere (Springer, 2005); R. Jahn, Chain Reaction (Arizona State University, 2007).
Akan Bassey Williams
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