Introduction

The heating of the earth–atmosphere system is accomplished via the processes of radiation, conduction, convection, and latent heat release. Radiation is energy in the form of electromagnetic waves that propagate through space; the waves travel at the speed of light and require no medium for propagation. Conduction is heat transfer via molecular collision, the essence of the process being an exchange of kinetic energy between molecules. Thus, it is most effective in a solid medium, in which molecules are relatively closely packed. Convection is heat transfer via mixing of parcels in fluids; it is much more rapid than conduction and acts on a larger scale. Since it requires translation of molecules, the convection process is limited to liquids and gases. Latent heat involves the molecular energy related to phase changes of water. For each gram of water, one calorie of heat is required to break the intermolecular bonds in the solid phase and vaporize the water. When water vapor condenses, this amount of heat is released. Thus, heat is transferred from the ground, where water is evaporated, to the atmosphere, where it condenses.

Over the earth as a whole and over time, the radiation received from the sun is largely equivalent to that emitted by the earth and atmosphere. This situation, termed the radiation balance, is required if the earth is to maintain a thermal equilibrium. For any given location, the radiation received at the surface is balanced by the remaining processes of heat transfer. This is referred to as the surface heat or energy balance. Atmospheric and surface characteristics determine the values of each term in the balance. The surface plays a particularly important role because the nature of the ground and the characteristics of the air layer just above it govern the fluxes that determine the heat balance. Latent heating intimately links the heat and water balances.