Heat is defined as energy that is in transit from a high temperature object to a low temperature one, and is an important variable in thermodynamics. The transfer of heat from one material to another is known as convection. Sometimes it is beneficial to minimize convection and in these circumstances Newton's law of cooling states that the heat transfer coefficient of the material should be as low as possible. The heat transfer coefficient is measured in special units of watts per square meter degrees Celsius. This unit is a measure of the heat energy that needs to be transferred in one second, over one square meter, to change the material by 1 degree Celsius (1.8 degrees Fahrenheit).

## Wood

Wood is a material that is produced by trees and is composed primarily of cellulose and lignin. Wood has a very low heat transfer coefficient of 0.13 watts per square meter degree Celsius. A 1-kilogram slab of wood will cool from 50 degrees Celsius (104 degrees Fahrenheit) to 20 degrees Celsius (68 degrees Fahrenheit) in 2 hours, 20 minutes.

## Sand

Sand is a sedimentary material that is composed of the compound silicon dioxide. It is naturally occurring and can be found on beaches all over the world. Sand has a very low heat transfer coefficient of 0.06 watts per square meter degree Celsius. This means it can retain heat for very long periods of time, and explains why the sand on the beach of a hot country is often still warm hours after sunset. A 1-kilogram container of sand will cool from 50 degrees Celsius (104 degrees Fahrenheit) to 20 degrees Celsius (68 degrees Fahrenheit) in 5 hours, 30 minutes.

## Expanded Polystyrene

Expanded polystyrene is a material used in packaging goods, and also in the construction industry. It has a very low heat transfer coefficient of 0.03 watts per square meter degree Celsius. This makes it an excellent thermal insulation in construction. A 1-kilogram block of polystyrene will cool from 50 degrees Celsius (104 degrees Fahrenheit) to 20 degrees Celsius (68 degrees Fahrenheit) in 11 hours, 20 minutes.

## Air

The Earth's air is composed of 78 percent nitrogen, 21 percent oxygen, 0.03 percent carbon dioxide and other trace gases. Fortunately, air can retain heat for many hours after being heated, and it is this fact that allows our houses to remain warm after the central heating switches off. Air has a heat transfer coefficient of 0.024 watts per square meter degree Celsius. A 1-kilogram container of air will cool from 50 degrees Celsius (104 degrees Fahrenheit) to 20 degrees Celsius (68 degrees Fahrenheit) in 14 hours, 15 minutes.