On Oct 12, 9:11*pm, wrote:
On Oct 9, 8:37*pm, Frank wrote:



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

Still air, like in a double pane window or fiberglass insulation,
seems to block heat tranfer. *Moving air, like a car radiator or a
breeze, *seems to heat the air and carry away heat. *Neither the long
wave radition or conduction seems to explain how air is heated. *As
you appoach a hot object with your hand, you feel radiation from the
hot object, not hot air. *It seems that air velocity and mixing is the
key to heating air ? * Dry air seems to heat much faster than damp
air?

Bill Snead

Think of the shimmer when looking over hot ground in the summer. This
is the heating process in action. The shimmer is caused because the
first foot or two of air adjacent to the ground is much warmer than
the air
above it, and because of the temperature difference it has a different
refractive index (just like the pencil sticking out of a glass of
water looks bent).

Some amount of wind is helpful because
it will push the warmer air up against a wick of some type, which will
break the surface tension and allow the heated air to travel upwards
(e.g. cell phone tower, ridge, line of trees, even a body of water).

I think the issue with damp air is really damp ground. Water has a
much
higher heat content factor, i.e. it takes more energy to raise the
temperature
of water a degree than it does dry ground. Moist air is actually more
buoyant
because H2O molecules are lighter than N2 or O2 molecules but still
take
up the same room. Normally this is only a factor when other lifting
mechanisms are at work (because of damp ground not heating as well)
but it has a huge effect in creating storms.

-- Matt