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 Concepts::

  1. Good Absorber is a good emitter
  2. It acts as an absorber when at lower temperatures and as an emitter when at higher temperature than surroundings

But I learnt in Geography that the temperature of a place is due to 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 text book 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)}]

Kirchoff's Law


1 Answer 1


A lot depends on cloud cover and moisture in the air. At night if the sky is clear, the black body radiation can be radiated into space through the water transmission window (where the water vapor is not absorbing in the IR). This will cool the sand. If there is cloud cover the radiation will be scattered back towards the earth. In that case the cooling effect won’t be as large.

The cooling by radiation can be significant and has been used in deserts to make ice, or help structures store ice. There are people who do research trying to use the night sky as a heat sink for a heat engine to improve the efficiency of HVACs.


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