Climate. – Sweden climatologically belongs to the Atlantic region. The Baltic Sea and the proximity of the Atlantic Ocean give it a special climatic character; more coninentale of the coastal countries of the Atlantic, p. ex. of Denmark and Norway, more maritime than the great Russo-Siberian region. The temperature shows positive anomalies of 5-10 °, with an average annual temperature ranging from 7 ° in the south, to −1 ° in the north, naturally with considerable fluctuations from year to year. Winter shows the strongest anomalies, summer the minor; in addition to this the anomalies are greater in the northernmost parts. Regions of a mainly continental character appear in the interior of Norrland, south and north of the great depression of Trondheim (see Scandinavia) – which allows a more free passage to the Atlantic winds – and, in relation to the surrounding regions, the plateau of southern Sweden. An absolute maximum temperature of 35 ° is reached during the short summer both in the interior of Norrland and in the rest of Sweden (the highest absolute temperature, 38 ° was recorded on 9 July 1933 in Ultuna, in Uppland). The number of days with temperatures above 25 °, ie the number of summer days, varies from 10 to 20 and is greater in the interior of southern Sweden. The absolute minimum temperature reaches −15 ° -20 ° in southern Sweden, up to −30-40 ° in Norrland (the minimum temperature was recorded on February 14, 1881 in Sveg, in Härjedalen, with −49 °). The number of days with a temperature lower than −10 ° can reach 140 and is maximum in the interior of Norrland, where the annual excursion is also maximum (25 ° -28 °); but if we take into account the absolute minimums and maximums of the temperature, it has doubled. The absolute daytime excursion fluctuates between 15 ° and 20 ° and is maximum in the interior of Norrland and in the interior of southern Sweden, that is in the regions with a distinctly continental character. Only in very severe winters does ice disturb marine traffic on the western and southern coasts of Sweden; in some years, however, the whole Öresund and a large part of the southern Baltic Sea may be covered with ice fields (Packeis) and floating ice. In the Gulf of Bothnia, on the other hand, which is generally covered with ice from the beginning of November to the end of May, it is only in very mild winters that traffic can continue unhindered. In very severe winters, the Åland and Kvarken seas are covered with a layer of ice that can be crossed by vehicles. The ports of southern Sweden on the Baltic are generally closed by ice from the end of November to the beginning of May.
According to directoryaah, the very irregularly distributed annual rainfall varies from less than 400 mm. to more than 1500 mm., indeed in some localities of the northern Alps they probably reach more than 3000 mm. Precipitation peaks along the northern Alps and along the western slopes of the southern Swedish plateau. These two regions are exposed to the damp winds of the Atlantic Ocean and rainfall generally increases with altitude: in southern Sweden it increases from 100 to 150 mm. every 100 meters; in the north, with many variations, they grow by over 200 mm. for 100 meters. Overall, rainfall decreases towards the east and is minimal in the interior of Norrland – especially in the north (less than 400 mm.) – which is sheltered from the rains,
The distribution of rainfall throughout the year is also very uneven. From February to March the driest period generally lasts, while July and August, and generally autumn, are the wettest periods. In continental regions, about one third of rainfall falls in July-August, and only one-tenth or less in February-March. The difference is greater in the more continental regions, and maximum in Karesuando in the north (40 and 6% respectively); in the maritime regions, on the other hand, rainfall is distributed fairly regularly – at the borders of the kingdom (observatory in Norrland) respectively 15 and 18%.
A significant part of the precipitation falls in the form of snow, and the more the further north we go (10% in southern Scania and more than 70% in north-western Norrland). In the northernmost regions this snow remains for eight months, in Norrland generally for six months. In southern Sweden, snow cover lasts a maximum of two to three months.
Hydrography. – More than 8% of Sweden’s land area is occupied by water in the form of lakes and rivers. These emanate a large number of river basins generally of small extension and often with indeterminate boundaries since numerous bifurcations are encountered and the watersheds often pass through vast areas of marshes. The perturbations of the glacial period gave Sweden its hydrographic imprint: the uncertain boundaries of the river basins, the richness of the lakes, surpassed only by Finland and Canada, the imperfectly developed longitudinal profile of its rivers, with numerous waterfalls and rapids that they place Sweden in first place among all countries in terms of hydraulic power.
In the lowlands of central Sweden, large areas are occupied by the great lakes, the Väner, the Vätter, the Hjälmar, the Mälar and infinitely smaller ones in shallow rocky basins, in tectonic cavities and in valleys barred by moraines. Rich in lakes is also the marginal area between the Alps in the west and the plateau of northern Sweden, where a row of elongated but almost always deep marginal lakes gives the landscape a characteristic imprint (see Scandinavia). A third region rich in lakes is the interior plateau of southern Sweden. The lowlands made up of primitive rocks and the Cambrosiluric and Cretaceous regions of southern Sweden are mainly poor in lakes.
Swedish rivers experience large swings in flow. Since snow accumulated during the winter is the main factor in this phenomenon, conditions in the north and south are completely different. As a rule, the melting of snow in spring and early summer causes one or more periods of flood, while autumn with its maximum precipitation causes a secondary one. Late winter and summer are lean periods as a result of low rainfall, low winter temperatures and intense summer evaporation. The winter lean is lower than the summer lean and becomes relatively lower towards the north, just as the flood is greater in the first summer. The backfilling of lakes in the spring and the accumulation of snow in the winter act as regulators. The difference between full and lean varies for the alpine rivers of Norrland from 4 to 6 meters, but further south it is very small, albeit with great differences between the various rivers. For example, the rivers of the Scania plain show a distinctly southern character with lean in summer and full in winter.
The runoff coefficient ranges from 75% in the north to 30 or 40% for the lowland rivers of central Sweden and Scania. The alpine rivers of Upper Sweden have the greatest water flow and are fed by the great rainfall of the mountains.
The rivers of Sweden, especially the northern ones, are of great importance as a means of floating for timber to the coastal sawmills. Due to the frequency of jumps and rapids, they develop hydraulic power which, if fully used, would reach 18 million horses, of which 85% in the rivers of northern Sweden. Since the northern rivers have a noticeable low winter, so the filling of the lakes is very important for them, and this is increased and enlarged in the north with notable embankments. Only 10 or 15% of the available energy has so far been used.