As Earth rotates on its axis and revolves around the Sun, the basic geometry determines an excess of radiation from the Sun is received in the tropics, and there is a deficit in polar regions (Fig. 1.4). Contrasts in temperatures between the two regions would be far greater than observed (Chapter 5) were it not for the dynamic transport of energy polewards by the atmosphere and ocean. The contrast is greatest in winter, when the polar night sets in, with zero incoming radiation in the Arctic or Antarctic, and the contrast in summer is less in the northern versus southern hemisphere owing to the location of large continental land masses in mid- to high latitudes that more readily warm up than the oceans do. How and where the poleward energy transports occur greatly affect local climates.