The January mean temperatures in the southern portions of the continental interiors average about 50° or 60°. In summer the northern continental interiors are warm, with July means of 60° and thereabouts. These temperatures are not much higher than those on the west coasts, but as the northern interior winters are much colder than those on the coasts, the interior ranges are very large. Mean maximum temperatures of 86° occur beyond the Arctic circle in north-eastern Siberia, and beyond latitude 60° in North America. In spite of the extreme winter cold, agriculture extends remarkably far north in these regions, because of the warm, though short, summers, with favourable rainfall distribution. The summer heat is sufficient to thaw the upper surface of the frozen ground, and vegetation prospers for its short season. At this time great stretches of flat surface become swamps. The southern interiors have torrid heat in summer, temperatures of over 90° being recorded in the south-western United States and in southern Asia. In these districts the diurnal ranges of temperature are very large, often exceeding 40°, and the mean maxima exceed 110°.
The winter maximum rainfall of the west coasts becomes a summer maximum in the interiors. The change is gradual in Europe, as was the change in temperature, but more sudden in North America. The curves for central Europe and for northern Asia illustrate these continental summer rains (see fig. 12). The summer maximum becomes more marked with the increasing continental character of the climate. There is also a well-marked decrease in the amount of rainfall inland. In western Europe the rainfall averages 20 to 30 in., with much larger amounts (reaching 80-100 in. and even more) on the bold west coasts, as in the British Isles and Scandinavia, where the moist Atlantic winds are deflected upwards, and also locally on mountain ranges, as on the Alps. There are small rainfalls (below 20 in.) in eastern Scandinavia and on the Iberian peninsula. Eastern Europe has generally less than 20 in., western Siberia about 15 in., and eastern Siberia about 10 in. In the southern part of the great overgrown continent of Asia an extended region of steppes and deserts, too far from the sea to receive sufficient precipitation, shut in, furthermore, by mountains, controlled in summer by drying northerly winds, receives less than 10 in. a year, and in places less than 5 in. In this interior district of Asia population is inevitably small and suffers under a condition of hopeless aridity.