Why is the air colder in the upper atmosphere?
The air is colder in the upper atmosphere because of fewer air molecules (e.g. oxygen and nitrogen). These molecules all have their own energy in the forms of translational, rotational, and vibrational energy. With fewer air molecules, there is less energy that can be transferred to other molecules in the form of heat. With less heat, temperatures are colder.
I haven't heard that explanation before, but it could be one factor. The major factor is that the atmosphere is heated from the ground up. Solar radiation passes through the atmosphere without much direct heating of the atmosphere (especially colder dryer air). It heats the earth's surface which in turn heats the air above it. Also descending air is heated due to compression at lower elevations according to Boyle's Law. Surfaces with high albedo such as snow or ice, reflect much of the solar radiation which in turn tends to sustain cold air masses over such surfaces.
I don't think this is correct. The hottest part of the atmosphere, by far, is the "thermosphere", which is up around the altitude of low orbit satellites. It is very very thin, and so the heat content is negligible. But the temperature can reach over 2000 C. This part of the atmosphere is heated from the top down. Heat and temperature are different.
SW VandeCarr is correct. The main part of the atmosphere is heated from the bottom up, and so there is falling temperature with altitude.
Cheers -- sylas
See these discussions.
http://www.physicalgeography.net/fundamentals/7b.html , which is cited in
The temperature of a gas is related to atomic or molecular speed, i.e. the kinetic energy of the atoms or molecules, although some energy is stored within the rotation of atoms and molecules, and internal vibration of molecules.
The density does affect the local rate of interaction (heating) of air molecules with sunlight, so the local heating from sunlight is less in the upper atmosphere than it is closer to the surface. On the other hand, in the ozone layer, heating via the absorption of UV is significant.
Temperature profile of atmosphere as function of elevation
Actually, a lost of that reflected heat is insulated in the lower troposphere.
Also, in the stratosphere, temperature actually increases with altitude.
Are you talking about reflected heat or reflected solar radiation? Most of the heat is generated when solar radiation is absorbed at the surface. I agree of much that heat may be trapped in the lower troposphere given cloud cover, but not so much in clear dry air.
Yes. I believe the coldest temperatures of the "standard atmosphere" are around 18,000 to 20,000 meters. Above this temperatures start to rise. Heating from the ground up only applies below this level.
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