MadSci Network: Physics

Re: What causes some soils/sands to lose heat more quickly than others?

Date: Thu Oct 22 20:37:30 1998
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 908054569.Ph

Hi Chelsea,

This is a question that might be better answered by a geologist or a soil 
engineer; however, I will talk about the physics involved in soil heat 
loss. Thermal conductivity and heat capacity are two things, which effect 
heat loss.  Thermal conductivity is a measure of how fast heat flows 
through an object.  Thermal energy (heat) causes atoms and molecules to 
move or vibrate.  The more heat, the faster the vibration.  When a hot 
vibrating atom comes into contact with another colder atom it transfers 
some of is energy to the colder atom.  After the collision, the hot atom 
moves slower and the colder atom moves faster.  Packing things closer 
together (increasing the density) increases the number of collisions 
between atoms and improves the thermal conductivity.  Moving things 
farther apart decreases the number of collisions and reduces the thermal 
conductivity.  In terms of soils, sand has a lot of space between grains 
(lower density).  Sand should be a poorer conductor of heat than clay, 
which is denser.  How tightly compacted the soil is will also effect the 
transfer of heat.  Loose soils tend to be better insulators (lower thermal 
conductivity) than tightly compacted soils.  

Heat capacity is the ability to store energy.  Heat capacity is also 
related to the density.  Each vibrating atom is storing energy.  A higher 
density material has more atoms in a given volume.  More vibrating atoms 
store more energy.  Water has a high heat capacity and is often used to 
store thermal energy.  Wet sand can store more energy than dry sand.  The 
water molecules fill the space between the grains of sand increasing the 
density of the sand.  The higher energy content means that it will take 
longer to cool down.  The higher density of wet sand also means that it 
can conduct heat faster than dry sand.  The water content of a soil thus 
has a large effect on how much energy is stored and how fast it is lost.

For more information try using the words soil engineer or soil engineering 
in your web search.  I found a lot of references, which I think you will 
find usefull.  The web is a wonderful place to find information.  
Congratulations on finding and good luck with that soil 
research project.

Bob Novak

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