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#11
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How does sun heat the air?
On Oct 9, 10:47*am, Tim Taylor wrote:
Evan, While the long wave does have a greater input back into the total atmosphere and to radiation back into space I think in the boundary layer conduction and convection are still the primary sources of heat transfer. * *Since the air is primarily oxygen and nitrogen that are both opaque to long waves as well it it mostly the other sources that generate thermals. Hey Tim, I think you'll find if you dig into this that it's long wave radiation, plus convection (thermals + wind) and evaporation/ condensation. Conduction is a bit player. See second two answers here, though I cannot vouch for numbers http://www.newton.dep.anl.gov/askasc...0/wea00082.htm -- the first response given is in error. There must be something better on the web, but I can't find it quickly. What's missing here is useful info on absorption coefficients and heat flux vs wavelength and water vapor content. But what I am given to understand here is that most of the heat energy that drives thermals we can use comes from long wave infrared transmitted from the heated surface and absorbed in the first few feet / tens of feet / hundreds of feet. I cheerfully admit I don't have the time to go get the details to back up my case Best, Evan Ludeman / T8 |
#12
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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 |
#13
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How does sun heat the air?
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 |
#14
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How does sun heat the air?
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 |
#15
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How does sun heat the air?
On Oct 13, 10:10*am, mattm wrote:
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- Hide quoted text - - Show quoted text - Maybe the solar energy primarly reacts with (is absorbed by ) liquid water. If there is a lot of water (the ocean a big lake), the mass of water takes the energy but does not heat the air. However, if the ground is what we think of dry, maybe the limited water grabs the solar energy in a way that causes the air to heat quickly. In central Texas in August typically the earth gets dry enough to wilt the plants. As the plants turn brown, the maximum temperature will jump from about 95 to maybe 102 F. The thermals will increase in strength from about 350 feet per minute to over 500 feet per minute. In our quarry the average moisture content of the crushed rock is about 6 percent. In the dry spells the moisture content of the rock never falls below about 3 percent. So there is still a lot of water around even though the place looks dry. Bill Snead |
#16
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How does sun heat the air?
I wonder if the sun does do this. right guys? Am i wrong? well the sun brings light down and produces the heat we have now.
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#17
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How does sun heat the air?
On Thursday, October 8, 2009 at 6:59:55 PM UTC-7, 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 The atmospheric isn't totally transparent to solar radiation, but absorbs a portion of it before it reaches Earth. Solar irradiance at the troposphere is 1366 W/m^2; by the time it reaches Earth it has decreased to about 1000 W/m^2. The difference of 366 W/m^2 goes into heating the atmosphere. Of course the ground reradiates infrared energy it absorbs back into the atmosphere. Tom |
#18
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How does sun heat the air?
On Thursday, October 8, 2009 at 7:59:55 PM UTC-6, 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 |
#19
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How does sun heat the air?
Not exactly transparent. The sky appears blue because air molecules and suspended particles absorb and scatter certain wavelengths of light. This energy is transduced from photons into heat. However, the majority of advection is due to the previous explanation. Not a physicist, so someone else take a stab.
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#20
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How does sun heat the air?
At 20:17 27 March 2017, Duster wrote:
Not exactly transparent. The sky appears blue because air molecules and sus= pended particles absorb and scatter certain wavelengths of light. This ener= gy is transduced from photons into heat. However, the majority of advection= is due to the previous explanation. Not a physicist, so someone else take = a stab. The downward energy radiated by the sun is short wave and not absorbed by the atmosphere. The upward energy radiated by the ground is long wave which is. |
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