Heat can be produced in an object by absorbing energy in the form ofradiation. The quantity of radiation absorbed determines how hot the object will get. Greater absorption results in greater warmth. Objects cool off when they emit radiation into their surroundings. An object's temperature can fluctuate over time due to changes in its radiation absorption and emission (Figure 6.1). If absorption exceeds radiation loss from emission, the object will heat up. An object cools down when radiation absorption is less than emission. When these two radiation transfer processes are in balance, the object's temperature will remain unchanged.

            The heating of our planet's ground surface, water bodies, and atmosphere is mainly controlled by the processes described above. The main power source for this heating is the Sun, which supplies an initial input of shortwave radiation. However, insolation is not the only type of radiation used to generate heat energy. Our planet also receives considerable quantities of longwave radiation, much of which comes from the greenhouse effect. The best measure of the energy available for heating is net radiation. Net radiation is determined by subtracting an object's emissions (outputs) of all types of radiation from the quantity of all wavelength radiation that is absorbed (inputs). In Chapter 5, we examined the patterns of net radiation available to the Earth's combined surface and atmosphere annually and for January and July (see section Global Radiation Transfers, Figures 5.32 and 5.33). We will see in this chapter that these patterns are closely correlated to spatial variations in surface air temperature. 

            The most common measurement of our planet's heat energy, recorded regularly, is surface air temperature. Surface air temperature readings are recorded at approximately 15,000 weather stations worldwide. These readings are then transmitted electronically and used to construct weather maps and forecasts. The routine measurement of surface air temperature is crucial for various human activities. 

FIGURE 6.1  Relationship between radiation absorption and emission and the heating and cooling of an object. The heating of an object can occur when the absorption of radiation exceeds the loss of radiation by emission. Cooling of an object can take place when radiation emission exceeds radiation absorption. If absorption and emission are in balance, the temperature of an object will remain constant. Image Copyright: Michael Pidwirny.