<p>It has been proposed to <a href="https://e360.yale.edu/features/urban-heat-can-white-roofs-help-cool-the-worlds-warming-cities" rel="nofollow noreferrer">paint dark rooftops and roads white</a> 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.</p>

<p>Conduction, convection and thermal radiation in sand has been studied by <a href="https://pumas.gsfc.nasa.gov/files/11_14_07_1.doc" rel="nofollow noreferrer">NASA</a>. See also <a href="http://www.madsci.org/posts/archives/2003-10/1065121289.Es.r.html" rel="nofollow noreferrer">this post</a>.  </p>

<p>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.</p>

CC BY-SA 4.0
2019-06-29T02:38:39.107Z
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.
RBFOLLETT
CC BY-SA 4.0
2019-06-29T02:53:31.870Z
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.
Keith McClary
CC BY-SA 4.0
2019-06-29T17:19:26.200Z
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.
RBFOLLETT
CC BY-SA 4.0
2019-06-28T15:03:48.363Z
2019-07-02T01:38:42.393Z
2019-07-02T01:38:42.393Z
Keith McClary
<p>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.</p>

<p>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.</p>

<p>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?</p>

CC BY-SA 4.0
2019-06-28T00:17:57.030Z
Could you please cut down on your wall of text a bit? No one's going to read this in the current form.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-06-28T05:37:41.067Z
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?
Mark
CC BY-SA 4.0
2019-06-28T07:44:32.667Z
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.
Michael Walsby
CC BY-SA 4.0
2019-06-28T13:10:46.020Z
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.
John
CC BY-SA 4.0
2019-06-28T20:48:23.103Z
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.
RBFOLLETT
CC BY-SA 4.0
2019-06-28T21:09:58.050Z
John.  Of course the suns rays are heating the sand and that heat will be transferred to some depth by grain to grain contact.  The surface area of the grains of sand to the air is significantly increased and heat transfer to the air becomes highly efficient just like a radiator does and simple convection takes over to move the air through the sand radiator.  Sealing the porosity of the surface massively eliminates the surface area of the sand grains below for heat conduction to the air.  Convection is therefore also reduced.
RBFOLLETT
<geology><climate-change>
Are sand deserts the greatest natural convection heaters on Earth?
RBFOLLETT
# Are sand deserts the greatest natural convection heaters on Earth?

(Asked by: username_0 on 2019-06-27)

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?

**Tags:** geology, climate-change

> **Comments:**
> **username_1**: Could you please cut down on your wall of text a bit? No one's going to read this in the current form.  
> **username_2**: 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?  
> **username_3**: 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.  
> **username_4**: 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.  
> **username_0**: 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.  
> **username_0**: username_4.  Of course the suns rays are heating the sand and that heat will be transferred to some depth by grain to grain contact.  The surface area of the grains of sand to the air is significantly increased and heat transfer to the air becomes highly efficient just like a radiator does and simple convection takes over to move the air through the sand radiator.  Sealing the porosity of the surface massively eliminates the surface area of the sand grains below for heat conduction to the air.  Convection is therefore also reduced.  

---

## 1 answers

### Answer by username_5 on 2019-06-28. Score: 0

It has been proposed to [paint dark rooftops and roads white](https://e360.yale.edu/features/urban-heat-can-white-roofs-help-cool-the-worlds-warming-cities) 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](https://pumas.gsfc.nasa.gov/files/11_14_07_1.doc). See also [this post](http://www.madsci.org/posts/archives/2003-10/1065121289.Es.r.html).

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.

> **Comments:**
> **username_0**: username_5.  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.  
> **username_5**: 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.  
> **username_0**: username_5.   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.  

---

