Re: Why is there such a large difference between sand and air temperature

Like so many questions, your question is simple to ask, and difficult to answer. There are several issues that determine the temperature of sand compared to the temperature of air. These include heat capacity, density, solar radiation absorption and emission, convective heat transfer, and conductive heat transfer. First lets examine heat capacity. A web site that shows the heat capacity of several materials is: http://www.fofweb.com/Subscription/Science/ Helicon.asp?SID=2&iPin=enweath1541

They show the heat capacity of sand as 0.84 J/gK, or it requires 0.84 joules of heat to increase the temperature of a gram of sand one degree. The heat capacity of air is 1.004 J/gK. So it takes a little more energy to heat a gram of air one degree than it takes to heat up a gram of sand. By comparison, it takes over 4 times as much energy to heat up water.

The second issue is density. Sand is relatively heavy, weighing about 2 grams per cubic centimeter. Air is 80% nitrogen; one mole weighs 28 grams and occupies 24 liters at room temperature. So one cubic centimeter of air weighs 0.0012 grams. So it doesn't take much energy to heat up air. Air in contact with the hot sand will heat up, cooling the sand a bit. And once air heats up, it's density decreases and it rises, being replaced by cooler air. When sand is cooled by the movement of air, it is referred to as convective heat transfer. Also, because of the low density of air, it can only transfer small quantities of heat from a hot surface to a cooler surface.

The next issue is solar heat absorption. Sunlight shines during the day, providing light and heat energy. Just as air is transparent to visual light, it is transparent to much of the solar radiation spectrum. Therefore, sunlight passes through air without heating it a lot. As the sun shines on the surface of the sand, the sand absorbs much more of the sunlight than the air and heats up. Unlike the air, the sand can't move around. The only way that the sand can get rid of the heat supplied by sunlight is by thermal conduction to the sand below it, and to the air surrounding each sand particle.

Particle to particle contact between sand particles isn't very good for transferring heat; each particle only contacts with a small area of the particles next to it. For the most part, each particle of sand is pretty well insulated from its neighbors by the surrounding layer of air. Therefore, the sand particles exposed to sunlight are going to get pretty hot, pretty fast. Wet sand, on the other hand, can absorb much more energy per degree change in temperature, and water is more efficient at conducting heat from the top surface of sand particles to the sand below the surface. The heat capacity of sand is similar to that of other ceramic materials (i.e. rocks). So the high temperature of sand isn't a strong function of its mineral content.
Admin note: you can observe how poor dry sand is at heat transfer by checking on a hot day that the layer of hot sand is very thin, and the sand gets much cooler just a few centimetres down.

At night, sand and other solid surfaces cool by radiation to space, along with conducting heat to the air. Space is close to -271 degrees C. So, if something has a high emissivity, it is possible for it to cool to a lower temperature than the air temperature. That is the reason that sometimes in the fall you will see a puddle of water that has frozen over, even though the air temperature didn't go down to freezing.

So sand is not a bad solar heat absorber, however, it is not easy to get heat in and out of a bed of sand. Larger rocks are often used for solar absorbers because it is much easier to add and remove heat by blowing air between the rocks.

Hope that helps explain why sand is hot in the day and may be cooler at night.