What process causes the sky (lower tropopause) to appear blue in colour?

Question

BlackTom AI · Accepted Answer

The question asks which process makes the sky appear blue in the region near the lower tropopause. 
Option 1: Scattering. This is the correct mechanism. Rayleigh scattering by molecules in Earth's atmosphere preferentially scatters shorter (blue) wavelengths of sunlight in all directions, giving the sky its characteristic blue color during daylight. The scattered blue light comes from every part of the sky, which is why we perceive a blue dome overhead. 
Option 2: Reflection. Sky color is not primarily due to reflection off objects or surfaces; there is no large, uniform reflecting surface overhead causing the sky to appear blue. Reflection would typically require a distinct surface or object to reflect sunlight, which is not the case for the atmosphere itself. 
Option 3: Diffuse radiation. While the atmosphere emits and scatters light in many directions, the term diffuse radiation is not the explanatory mechanism for why the sky is blue. The color comes specifically from scattering of the incoming sunlight, not a separate diffuse emission process. 
Option 4: Absorption of blue wavelengths of visible light. If the atmosphere absorbed blue light strongly, the sky would appear the opposite color (more red/orange) due to missing blue wavelengths. In reality, the blue color arises because blue light is scattered out of the direct beam and into many directions, not because blue light is absorbed. The absorption story would not produce a blue sky.

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