Why is the Earth generally hotter near the Equator than at the poles? Why is it generally hotter in summer than in winter, especially outside the tropics? Would this be true on other planets as well? How would the pattern change over time, as features of the planet's orbit vary? Would a very slowly rotating planet lose its atmosphere to condensation on the nightside? Would a planet whose rotation axis was steeply inclined relative to the normal to the plane of the orbit, or a planet in a highly elliptical orbit, have such an extreme seasonal cycle that it would be uninhabitable? The answers to these questions are to be found in the way the geographic and temporal pattern of illumination of the planet plays off against the thermal response time of the atmosphere, ocean, and solid surface of the planet. Generally speaking, in this section we seek to understand the features of a planet that determine the magnitude and pattern of geographic and seasonal variations in temperature.
Most of the discussion of temporal variability will focus on seasonal rather than diurnal variations, but much of the same considerations apply to both cycles, and so some remarks will be offered on the diurnal cycle as well. It should be kept in mind that the distinction between diurnal and seasonal cycle is meaningful only for bodies such as the Earth, Mars, or Titan whose rotation period is short compared with the period of orbit about the Sun.