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. Convection is much more important than conduction in transporting heat within the troposphere. Heat is also transported horizontally within the atmosphere and the process is called advection . The process of heat transport by both conduction and convection is known as sensible heating . No mass is exchanged and no medium is required.