Because of maritime influences, particularly the warm North Atlantic Drift and the prevailing westerly airstreams, Sweden has higher temperatures than its northerly latitude would suggest. Stockholm averages –3° C (26° F ) in February and 18° C (64° F ) in July. As would be expected from its latitudinal extent, there is a wide divergence of climate between northern and southern Sweden: the north has a winter of more than seven months and a summer of less than three, while Skane in the south has a winter of about two months and a summer of more than four. The increasing shortness of summer northward is partly compensated for by comparatively high summer temperatures, the greater length of day, and the infrequency of summer cloud; the considerable cloud cover in winter reduces heat loss by radiation. Annual rainfall averages 61 cm (24 in) and is heaviest in the southwest and along the frontier between Norrland and Norway; the average rainfall for Lapland is about 30 cm (12 in) a year. The maximum rainfall occurs in late summer, and the minimum in early spring. There is considerable snowfall, and in the north snow remains on the ground for about half the year. Ice conditions in the surrounding seas, especially the Gulf of Bothnia, often are severe in winter and seriously interfere with navigation.