Ask any climate scientist the question: "Why are deserts so hot in the day time and so cold at night" and generally you will get the same answer, it’s all about the lack of moisture and latent heat and the greenhouse gas effect. Consider however for example, a simple household convection heater that uses the principles of: a radiator, conduction and convection to warm a house. The radiator fins greatly increase the surface area of the heat source to significantly increase the conduction of heat to the air and the basic concept of convection circulated the warm air upward. Very efficient, and control is simply to turn the element on and off with a thermostat. A less used method of control however, but still effective, is simply to block the fins so conduction and convection is reduced.

Enter Mother Nature, with sand deserts covering vast surfaces of the earth. Any geologist will tell you that Aeolian sand deposits can have porosities upwards of over 45%, permeability is unlimited and finally the surface area of the sand grains (on and below the surface) for heat conduction is hundreds of times greater than just a flat non porous surface. The Sahara Desert covers 3.5 million square miles of Aeolian sand deposits. The sun supplies the heat to the sand both at the surface and below (surface conduction grain to grain), the sand grains are the radiator fins that conduct heat to the air and finally convection looks after the rest. Shut off the sun every night and the sand cools very quickly because of the increased conduction and convection soon replaces the hot air with cold.

Most of Europe right now is experiencing a massive heat wave coming from the Sahara Desert to the south and of course blames global warming. The question is "Are Sand Deserts The Greatest Natural Convection Heaters On Earth" and can we control them? We cannot control the sun, but can we not close some of the fins on these "desert convection heaters", perhaps by sealing some of the surfaces of the desert sands with tarps or liquid sealers? Anything that stops the convection will reduce the heat output. Has Mother Nature in the past, been playing with the Earth's thermostat simply by reducing and increasing the size of our great deserts?

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    $\begingroup$ Could you please cut down on your wall of text a bit? No one's going to read this in the current form. $\endgroup$ Jun 28 '19 at 0:17
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    $\begingroup$ Your question assumes there is significant airflow through the sand. Do you have any evidence to support this? Any particular reason why the air would decide to travel down into the sand, absorb heat, and then flow back out into the atmosphere? $\endgroup$
    – Mark
    Jun 28 '19 at 5:37
  • $\begingroup$ The measures you propose are unthinkable. Environmentalists would have a fit, there would be riots in Trafalgar square at the idea of thousands of square miles of pristine desert being covered over with some sort of sealant. $\endgroup$ Jun 28 '19 at 7:44
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    $\begingroup$ You do realize the heat warming the sand is coming from the sun and not the sand itself right? covering the sand just means the heat is absorbed by whatever covers the sand instead. $\endgroup$
    – John
    Jun 28 '19 at 13:10
  • $\begingroup$ Mark. The porosity of the sand is over 40% with unlimited permeability for the air to travel through it and can be described as a radiator with sand grains as the heat exchanger fins. Even if the sun’s energy only heated up the top 6 inches of the sand, the surface area of the sand grains for heat conduction to the air in that top 6 inches is massive compared just the desert surface. The minute the air within the sand starts to warm, it will rise up and be replaced by the cooler air above, hence convection. I am talking about the extreme efficiency of a porous surface to conduct heat. $\endgroup$
    Jun 28 '19 at 20:48

It has been proposed to paint dark rooftops and roads white to reflect solar energy back to space. This should also work for deserts, although sand already has a high albedo. Applying a sealer of the same colour would not make much difference.

Conduction, convection and thermal radiation in sand has been studied by NASA. See also this post.

In daytime the sand is hottest at the surface and cooler below. If it is a horizontal surface there won't be convection, since hot air rises. Possibly there could be convection with uneven surfaces (dunes). I think petroleum geologists know how to calculate flow given permeability and pressure gradient.

  • $\begingroup$ Keith. Actually I am not talking about changing the albedo of the sand, just shutting down the convection of air through the hot sand below the surface. Take a square meter of asphalt and you have a square meter of surface area to conduct heat into the air, take a square meter of sand and you have 100 times more surface area of the sand grains below the surface to conduct the heat to the air because of the porosity. Sealing the surface just stops the flow of air through the sand and reduces the convection. $\endgroup$
    Jun 29 '19 at 2:38
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    $\begingroup$ I have never heard that sand behaves in a radically different way from solid materials as you suppose. If it did, I think the effect would have been noticed and studied. $\endgroup$ Jun 29 '19 at 2:53
  • $\begingroup$ Keith. Too many scientists are completely focussed on greenhouse gases causing global warming to grasp another concept. As a Geologist of some 46 years I can assure you that natural gas flows through solidified sandstone, and air will flow easily through unconsolidated sand. A velocity meter held over a solid surface and compared to measurements over sand could easily prove convection is happening. $\endgroup$
    Jun 29 '19 at 17:19

1.5° to 3° can be linked to anthropogenic global warming. https://www.worldweatherattribution.org/wp-content/uploads/July2019heatwave.pdf

This one, and others before, would not have been possible to that degree without anthopogenic heating. The desert is not to blame, they actually have, as has been said, a pretty high albedo and clear skies. During the day it gets hot because insolation, but during night the heat is qickly radiated out into space. There can be a hefty diurnal temperature range at the surface.

A a general rule in meteorology, the air cannot go into the ground. It is only the surface that is directly exposed to and heated by the sun during day. At 10cm depth already (in a mobile dune) the temperature plays a minor role. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013JF002839

So, no, deserts are no natural convection heaters. The Sahara air actually suppresses convection frequently forming rock stable inversions. Some deserts are extreme coolers (Gobi, Antarctica, Greenland's dwindling ice sheet, ice covered mountain tops) due to their high albedo and radiation of the daily received insolation out into space. Changing albedo (e.g. by thawing) exposes more heatable surface to the sun, leading to a runaway feedback.

  • $\begingroup$ Would be nice to get a comment with the downvote ... $\endgroup$
    – user18411
    Dec 8 '19 at 10:33

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