The preceding discussion has revealed two limiting forms of behavior a planet can exhibit in the course of its seasonal cycle. A "waterworld," having high thermal inertia in the ocean-atmosphere system, responds primarily to the annual average insolation. Such a world will be coldest at the poles and warmest at the equator, unless the obliquity exceeds about 54o, in which case the warmest climates will be found near the two poles. A "desertworld," having little thermal inertia in either the surface or the atmosphere, responds to the instantaneous insolation at each time of the year. The location of the highest temperature moves from some latitude North of the equator to the same latituded South of the equator, and back again, in the course of the year. For small obliquity, the poles are frigid throughout the year, and the hot spot executes modest excursions about the equator. For obliquities greater than about 18o, the excursion goes all the way from pole to pole, assuming a uniform albedo. Geographical and temporal albedo variations alter this picture. Formation of permanent ice or snow cover near the poles will tend to keep the polar regions cold throughout the year; this effect is assisted by the thermal inertia implied by the latent heat required to melt or sublimate ice, which limits the summertime temperature increase. The Earth shows some characteristics of both limiting cases, with extreme continental climates and equable maritime climates. Thermal inertia sufficient to moderate the seasonal cycle can be provided either by a thick atmosphere or a well mixed liquid layer at the surface, which need only have a depth of some tens of meters. Heat storage provided by non-melting solid surfaces is almost never sufficient to have a significant affect on the seasonal cycle, though it can substantially moderate the diurnal cycle for planets with rotation periods on the order of a few Earth days or less.
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