Application Of Kirchoff Law (Desert)

[Aurelius120]

Homework Statement:: Sand is rough and black so it is a good absorber and radiator of heat depending on temperature.
During the day, sand's radiation of the sun's energy superheats the air and causes temperatures to soar. But, at night most of the heat in the sand quickly radiates into the air and there is no sunlight to reheat it, leaving the sand and its surroundings colder than before
Relevant Equations:: Good Absorber is a good emitter
It acts as an absorber at low temperatures and as an emitter at higher temperature

But I learned in Geography that the temperature of a place is the temperature of the air around it
It said that the temperature of air is influenced by terrestrial radiation and never solar radiation
Which also explains the lapse of temperature with altitude and the inversion of temperature gradient at night(sometimes)

From that point of view

Terrestrial radiation from sand should actually warm the air up at night

I had this explanation
At day Sand heats up and acts as radiator making the days hot
And at night it should be cold because the sand acts as an absorber now absorbing all heat from the warm air

But this different from my textbook explanation
[{Which seems to suggest that it is the sand that is hot at day and cold at night and sand affects temperature(rather than air)}]

 

[hmmm27]

You going to believe a Geography course ?

It's all about equilibrium : if something has a temperature at all then it is radiating heat.

During the day the incoming solar radiation is enough to heat up the sand to the point where it's radiating as much power as it's receiving.

During the night, there's almost no radiation coming in from the sky, but the sand continues to radiate, so it loses heat and gets colder.

 

[jim mcnamara]

Kirchoff's Law of Thermal Radiation is what you are discussing, I believe. The thread has been moved to Earth Science for now. Let's see what answers we get.

What you are saying seems a little confused - to me.

 

[Tom.G]

Perhaps you have been outdoors on a cool but Sunny day and no wind. If you are in the shade you are chilly. But if you step into the Sunlight you are warmer... even comfortable.

If there are broken clouds and one of them blocks the Sun, you will feel cool again.

I'm sure the experts here will have a somewhat different, and technical, explanation, but...
Let's try this.

Anything above absolute zero (about -273°C) will radiate energy.

• You have two "things" at different temperatures.
• The hotter one will radiate more energy than the cooler one. (the Sun vs. Sand)
• Generally, the more opaque a thing is, and the more mass it has, the more energy it will absorb.
• Being rather transparent and very low mass, Air will not absorb very much energy.
• Sand, being opaque and higher mass, will absorb a noticeable amount of energy. (on a Sunny day, that makes it hotter than the air)

Space itself (outer space) has a temperature of about -270°C.

• At night, both the Sand and the Air radiate energy to Space.
• The Air, being lower mass, does not have much absorbed energy, and quickly cools.
• The Sand, with it's much higher absorbed energy, takes a bit longer to cool.

Terrestrial radiation from sand should actually warm the air up at night

Well, yes it does, by a small amount. But as noted above the Air, being mostly transparent, does not absorb much of the energy.

Hope this helps!

Cheers,
Tom

p.s. The amount of energy transferred between two objects depends strongly on the temperature difference between them. For instance a temperature difference of 10% between two objects causes 46% difference in energy transfer.
(the above from memory. the experts may well correct this number!)

 

[Baluncore]

That would be Kirchhoff, spelled with hh and ff.
https://en.wikipedia.org/wiki/Kirchhoff's_law_of_thermal_radiation

 

[jim mcnamara]

Thanks for the correction.

 

[Drakkith]

But I learned in Geography that the temperature of a place is the temperature of the air around it
It said that the temperature of air is influenced by terrestrial radiation and never solar radiation
Which also explains the lapse of temperature with altitude and the inversion of temperature gradient at night(sometimes)

Air temperature isn't affected that much by radiation from either the Sun or the Earth, as it's mostly transparent to both visual and infrared wavelengths, with exceptions being around specific absorption bands. I believe it's far more affected by ground temp and convection than by radiation.

I think it's only once you get high up into the atmosphere, well away from the ground and the convection cells that form, that radiation becomes the dominant method to cool or heat the air.