Energy Transfer and Thermal Equilibrium
A system can acquire or lose energy in four ways:
(1) Radiation - Radiation enters the system from its environment. Radiation is emitted by the system into its
environment.(2) Convection - The system exchanges fluids with its environment. The internal energy per unit mass
associated with the fluid lost and with the fluid gained determines the net change in the internal energy of
the system.(3) Conduction - Heat may be either gained or lost by the system depending upon its temperature and the
environmental temperature along the system's boundaries.(4) Work - If the system does net work on its environment, then it will lose energy. If the environment does
net work on the system, then the system will gain energy.Hence there are four ways a system can acquire more energy and four ways it can lose energy. If the gains exceed the losses, the system acquires net energy and its internal energy increases. This is usually accompanied by an increase in the average temperature of the system, but it can also induce an increase in latent heat through phase changes with no increase in temperature. If the losses exceed the gains, the system loses net energy and its internal energy decreases. This is usually accompanied by a decrease in the average temperature of the system, but it can also induce an decrease in latent heat through phase changes with no decrease in temperature. If the gains and losses are balanced, the system is said to be in "thermal equilibrium" and usually its temperature remains constant.
On a daily basis, the daily maximum and minimum temperatures (when the atmospheric temperature is constant, not getting warmer or colder) occur at times when the atmosphere is in thermal equilibrium. The same is true of the times of annual maximum and minimum temperatures, except that in those cases the equilibrium is averaged over a day's time, not instantaneous.