How does sun heat the air?

On Oct 8, 9:59*pm, " wrote:
The sun shines down through the atmosphere, hits things on the ground,
the things on the ground get hot, and then the hot things on the
ground heat the air?

Sun light passing through the air does not heat the air at all? *Heats
the air some but not much?

If the air is transparent to solar energy, how is *the air able to
pick up so much energy from hot objects on the ground?

Bill
6W

Bill,

Solar (electromagnetic) energy in the visible light spectrum doesn't
couple very well to dry air. However, energy in the visible light
spectrum *does* couple nicely to darker objects like a dark farm field
or a parking lot, etc. The visible light energy heats the ground,
thereby converting visible light energy to much lower frequency
(longer wavelength) IR energy, and energy in the IR band *does* couple
to dry air in close contact with the hot objects. However, dry air is
a great insulator, so only the air a few centimeters/meters above the
ground gets heated directly, forming a mat of heated air underneath
colder air above. This is an unstable configuration, and thermals are
nature's mechanism for re-establishing equilibrium. This mechanism is
closely analogous to heating a pot of water on the stove. The water
immediately adjacent to the bottom of the pot gets hot, and this sets
up circulation currents to re-establish equilibrium in the rest of the
pot. If the heating is fast enough, local vaporization takes place
causing bubbles to form (i.e. the pot boils).

Just as an evolutionary side note, eyes evolved to see in the visible
spectrum *because* the air is transparent (i.e. very low coupling
coefficient) to that band of energy. If our atmosphere happened to be
only transparent to what we call infrared, then we'd all see in the
infrared, not the visible range, and thermals would be much easier to
"see" ;-).

Regards,

TA