Wien's Displacement Law and reradiation of LWIR by C O 2 Consider Wien's Displacement Law (if I understand correctly): λ = b / T Where, λ = Peak Wavelength b = 0.028977 mK (Wien's constant) T = TemperatureAccording to this law C O 2 molecules can only absorb and reradiate the Long Wave I.R. frequency (radiated from Earth) in their 15 micrometre (μm) spectrum at a temperature of -80 degrees Celcius. The link with temperature seems crucial to me since the only part of our athmosphere cold enough for this to happen is the Mesosphere at about 50-80 km from Earth. Troposphere and Stratosphere are not cold enough (coldest temperature is about -55 degrees Celsius). In these parts of our atmosphere it is just not cold enough for C O 2 to reradiate I.R. wavelengths back to Earth. Then why is C O 2 considered a greenhouse gas in our Tropo- or Stratosphere? Am I missing out on something here?In the Mesosphere, however, temperature can drop to over -100 degrees Celsius. In this part of the atmosphere C O 2 can reradiate LWIR back to Earth. But the problem is that the air is so thin, there are hardly any molecules of C O 2 What am I missing here in my reasoning?

Question

Wien&#039;s Displacement Law and reradiation of LWIR by   C      O    2  Consider Wien&#039;s Displacement Law (if I understand correctly): λ = b / T Where, λ = Peak Wavelength b = 0.028977 mK (Wien&#039;s constant) T = TemperatureAccording to this law   C      O    2   molecules can only absorb and reradiate the Long Wave I.R. frequency (radiated from Earth) in their 15 micrometre (μm) spectrum at a temperature of -80 degrees Celcius. The link with temperature seems crucial to me since the only part of our athmosphere cold enough for this to happen is the Mesosphere at about 50-80 km from Earth. Troposphere and Stratosphere are not cold enough (coldest temperature is about -55 degrees Celsius). In these parts of our atmosphere it is just not cold enough for   C      O    2   to reradiate I.R. wavelengths back to Earth. Then why is   C      O    2   considered a greenhouse gas in our Tropo- or Stratosphere? Am I missing out on something here?In the Mesosphere, however, temperature can drop to over -100 degrees Celsius. In this part of the atmosphere   C      O    2   can reradiate LWIR back to Earth. But the problem is that the air is so thin, there are hardly any molecules of   C      O    2  What am I missing here in my reasoning?

Kyler Crawford · Accepted Answer

Wein&#039;s displacement law has to do with how the peak in a blackbody spectrum shifts with temperature. It does not have anything to do with whether a particular temperature is required for a substance to absorb or radiate at that wavelength.Carbon dioxide will both absorb and radiate at 15   μm in the both troposphere and stratosphere. If radiation at the correct wavelength exists, then CO2 molecules will absorb it. Usually, they get knocked around and lose the excess energy through collisions, thereby acting to increase the temperature of the atmosphere. But they can also be knocked into a higher energy level through collisions and then lose the energy through radiation in a random direction.

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