Why does temperature decrease in the mesosphere and increase in the thermosphere?

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1.Conduction, convection and radiation.

2.Height increases; Temperature decreases

3.Radiative Emission

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1.Conduction, convection and radiation.

• The portion of solar radiation received by the earth is called insolation, meaning "incoming solar radiation". 
• It is received in the form of short waves, and it warms up the earth’s surface. 
• The earth in turn gives out or radiates that heat it has absorbed back into the atmosphere in the form of a long wave radiation. This is known as terrestrial radiation
• The atmosphere allows the rays of the sun to pass through without being heated by them (85%). However, it gets warmed by the terrestrial radiation (85%). So, we see that the air around us does not get heated directly by the sun’s rays but indirectly by the heated surface of the earth through conduction and convention currents.
• Thus, it is clear that the atmosphere receives a larger part of its energy supply from the earth and not directly from the sun.
• The mesosphere (meaning "middle sphere") is the third layer in our atmosphere, occupying the region 50 km to 80 km above the surface of the Earth, above the troposphere and stratosphere, but below the thermosphere. 
• Because of it's altitude, the air is a lot thinner in the mesosphere than in the stratosphere below. There are fewer air molecules to absorb incoming electromagnetic radiation from the Sun.
• Conduction and convention current don't go up to that much height.
• Thus, temperature in the mesosphere keeps dropping with increase in altitude until it reaches about -100°C.
•  The mesosphere is the coldest of the atmospheric layers.
•  In fact, it is colder than the Polar regions of the Earth - cold enough to freeze water vapor into ice clouds.

2.Height increases; Temperature decreases [Exception: Thermosphere].

• The thermosphere (meaning "heat sphere") is the next layer of the atmosphere after the Mesosphere. 
• The thermosphere has extremely high temperatures. Within the thermosphere temperatures rise continually to well beyond 1000°C. 
• The few molecules that are present in the thermosphere receive extraordinary amounts of energy from the Sun, causing the layer to warm to such high temperatures. 
• The high temperature is due to the intense light rays, or radiation, from the sun. 
• Since there is little to no atmospheric gases above the thermosphere, there is no absorption of the heat from solar radiation, and so temperatures soar.

3. The CO2 releases some of energy as photons in a process called "radiative emission.

• Carbon dioxide in the mesosphere also helps keep this layer cold. 
• CO2 molecules absorb heat energy when they bounce off of other molecules. 
• The CO2 releases some of that energy as photons in a process called "radiative emission". 
• Some of those photons travel upwards, carrying heat away from the mesosphere. 
• Thus, temperature in the mesosphere keeps dropping with increase in altitude until it reaches about -100°C.

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