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8 HUGO SJORSFig. 9. The length of daylight,twilight and nocturnal darkness,as dependent on latitude, for eachmonth of the year. The twilightin winter is longer in practice dueto multiple reflexion between snowand sky. From Angstrom 1958.of Scandinavia, even though temperatures below-45°0 are exceptional. Low temperatures occur inparticular after large parts of the Bothnian watersand coastal parts elsewhere have bece>me frozenin late winter, but there is always enough openwater left in the Baltic proper to have a mitigativeeffect on the winter temperatures of coastal southeastSweden. Still more important, especially inthe south and south-west, where the coast hasmean temperatures about or only slightly belowfreezing even in January and February, are theeffects of frequent invasions of mild and moistAtlantic air. They reach a variable way to thenorth and exert an influence even in northernSweden, as evident from the fact that the wintersare far warmer over all Sweden than would beexpected from latitude.The winter climate has a strong south-to-northgradient, quite contrasting to the equalized meantemperatures in summer. But the gradient is notuniform, for there are secondary effects involvingdifferences between the milder coasts and the moresevere inland, and to some extent also betweenlowland and upland. Above all, in the shelter ofActa Phytogeogr. Suec. 50