Thermometers are located inside a white painted shelter mounted about 1.5 meter above the ground. The shelter should be located in an open grass area away from trees, buildings, and other obstacles. It should be no closer than four times the height of the nearest obstacle. In response to temperature gradients, heat is transported via conduction, convection, and radiation following second order of thermodynamics , which states that all systems tend toward disorder. No mass is exchanged, but a medium is required.
Metals are excellent conductors of heat, whereas air is very poor conductor of heat.
A fresh snow cover has extremely low heat conductivity because of the air trapped between individual snowflakes. Therefore, a thick snow cover can prevent freezing of the underlying soil even though the temperature of the overlying air may drop well below freezing. Mass is exchanged and a medium is required. Convection occurs as a result of air density difference within the atmosphere. A convective circulation of air transports heat vertically from the earth's surface into the troposphere.