When we turn our face towards the sun on a day with clear skies: Is the heat we feel on our skin actual heat radiation from the surface of the sun or do we just feel the warmth of the molecules in our atmosphere which have been “warmed” by radiation from the upper atmosphere?
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Both, kind of. Let’s unpack that.
First fact to know – heat flows from warmer body to colder body.
Second fact: There are 3 ways to transfer heat: radiation, conduction, and convection. Conduction is flow of heat through the material. Not relevant here. Convection is transfer of heat between a surface of an object and the surrounding fluid (i.e. transfer from your body to air or vice versa), and radiation is heat transfer via electromagnetic waves.
When you’re standing in the sun, you’re being warmed directly by actual radiation from the sun.
Additionally, you’re feeling the heat, not from the atmosphere, but from your own body. Your body is at 37°C. A warm summer day is, say, 30°C. That means air is colder, and heat actually transfers from your body to the air. You’re giving heat to the air. But, since your body is constantly producing heat due to various biochemical processes, the warmer the air is, the less heat you’re able to dissipate (because the speed of heat transfer depends on the difference in temperatures), the warmer you feel.
So, TLDR, you’re being warmed by the sun and by your own body.
Btw., a practical application of your question – because the sun has the ability to directly heat up objects on Earth, that’s the reason why, if you want to measure air temperature, you put the thermometer in the shade, not on direct sun. You want to measure how warm the air is without adding in additional heat from the sun.
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