Climate
Climate describes the long-term average of weather conditions (often spanning at least 30 years) for a specific area. It primarily covers temperature and precipitation, but also includes wind, humidity, atmospheric pressure, solar radiation, and other meteorological factors.
About Climate
Different regions on Earth or within individual countries can be divided into climate zones, where each zone shares a similar climate. For larger global areas, a climate classification system known as the Köppen system is generally used. The 30-year intervals frequently utilized to describe a climate are referred to as normal periods. The reference period (standard normal period) currently used in Sweden is 1991–2020.
Sweden also exhibits regional climate differences and can thus be divided into distinct climate zones. Within Swedish forestry, a national climate zonation system (Odin et al., 1983) is frequently used, which is based on temperature sums (see below) and elevation above sea level.
Map from Lundmark (1988)
Climate zonation provides a broad classification of the climate. Within these climate zones, local conditions can vary significantly due to factors such as ocean currents, distance to coastlines and large lakes, and altitude.
Areas positioned in the lee of the Scandinavian mountain range feature a local continental climate. This is characterized by greater temperature and precipitation differences between summer and winter, along with relatively low precipitation compared to coastal regions. Along the west coast and parts of the east coast, the climate is more maritime, distinguished by narrower temperature gaps between summer and winter. The ocean's stabilizing influence is present not only along the coasts but also in western Jämtland, where warmer, more humid air masses from the Atlantic break through the mountain chain. This climate zonation is clearly visible in specific maps showcasing mean temperature, temperature sums, and the length of the growing season.
The map shows areas that deviate positively (continental climate influence) or negatively (maritime climate influence) from the temperature climate that would be expected based solely on latitude and altitude.
Mean value of actual annual precipitation for the standard normal period 1991-2020. Map from SMHI.
The prevailing wind direction in Sweden is between the southwest and west. The moist air masses pushed into Scandinavia face a major barrier in the mountain range. As the air rises and cools, moisture drops out in the form of rain or snow. Combined with low summer temperatures in alpine areas, these high precipitation volumes create ideal conditions for glacier formation.
The Swedish west coast lacks the rain-shielding protection of a mountain range, resulting in relatively heavy precipitation and a distinctly maritime climate. When these moist air masses reach the western slopes of the South Swedish Highlands (Småländska höglandet), they trigger the same orographic effect seen in the northern mountains, giving this region the highest precipitation levels in southern Sweden. Conversely, the east coast lies in a rain shadow, receiving small amounts of precipitation compared to the west coast.
Humidity = Precipitation - Evaporation
Map: Humidity (1951-1980). From Lundmark (1986).
reas where water input via precipitation exceeds the combined total of evaporation and plant transpiration are classified as humid. Humidity plays a vital role in determining whether vegetation risks suffering from water scarcity. During the winter period, vegetation effectively absorbs no water, experiencing a form of dry spell regardless of how much precipitation falls. Consequently, humidity is of greatest significance during the growing season.
The highest humidity levels are found in regions with high precipitation and/or low temperatures, conditions typical of Sweden's western and northern areas. In the eastern parts of the country, precipitation is lower while temperatures are higher, resulting in low humidity. In these eastern regions, humidity during the growing season is negative, meaning the water deficits that develop over the summer must be replenished during the winter. For these reasons, dry spells are more frequent in eastern Sweden. However, western Sweden can suffer just as extensive damage from severe droughts because its trees are less adapted to water-scarce conditions.
Map: Mean length of the growing season the standard normal period 1991-2020. From SMHI.
The growing season is defined as the period of the year when the daily mean temperature exceeds +5°C for four consecutive days. Around this temperature in the spring, the ground is thawed enough for root activity in trees to begin, and at a corresponding temperature in the autumn, growth halts in preparation for winter.
Changes in the length of the growing season largely align with variations in the annual mean temperature from north to south. However, the exact start and end dates can vary even within regions that share the same overall growing season duration. This is due to the impact of large lakes and oceans, which warm up slowly—exerting a cooling effect in spring and a warming effect in autumn.
Turbulent air currents moving over Sweden, mainly from the southwest, cause substantial fluctuations in weather, altering the start and end dates of the growing season from year to year. This variance is most pronounced in the southern parts of the country, which lack the protective barrier of the mountain range. Due to the vast climate differences between south and north, the growing season in southernmost Skåne is twice as long as it is in the far north. The impact of altitude is also highly visible in the South Swedish Highlands, which experience a markedly shorter growing season than the surrounding lowlands of Götaland.
The map applies to corrected precipitation 1951-1980 and comes from Lundmark (1986).
The temperature sum is the accumulated daily mean temperature above +5°C during the growing season. This metric effectively describes how warm the growing season actually is. Along the coasts of Götaland, the growing season is longer on the western side. However, the temperature sum remains roughly equal because the east coast compensates for its slightly shorter growing season with drier, warmer summers.
A favorable temperature climate also occurs along the Norrland coast. Right by the shores of the Baltic Sea, however, the cooling sea air results in a slightly lower temperature sum. Maps of temperature sums also reveal that the area surrounding lakes Mälaren and Hjälmaren enjoys a highly favorable temperature climate during the growing season. On average, the temperature sum decreases by 58 degree-days for every degree of latitude north, and by 90 degree-days for every 100-meter increase in altitude.
The warming effect of water in autumn is clearly observable along coastlines and around larger lakes, as well as in Jämtland, which is influenced by warming Atlantic winds. In contrast, local continental areas like the South Swedish Highlands and the inland regions of northern Sweden face early autumn frosts, which also contributes to a shorter growing season.
Map from SMHI
The isolines (contour lines) for the final spring frosts generally track those of the growing season's length. In southern Sweden, frost ceases early along the west coast and around lakes Vänern and Vättern. Due to continental climate effects, frost occurs relatively late on the South Swedish Highlands. The Baltic Sea exerts a stabilizing effect on the absolute outermost coastal strip, while the areas just inland suffer late spring frosts. In northern Sweden, the conclusion of spring frost follows latitude and altitude closely. The influence of westerly winds brings a relatively early end to spring frosts in the Storsjön lake district.
Map from SMHI
From Warm-Temperate to Cool-Temperate Zone
The southernmost parts of Sweden lie within the warm-temperate zone, where the summer period is relatively warm and long, and winters are not overly severe. This area features a high proportion of deciduous trees, including several "noble" (broadleaf) species such as oak, beech, elm, linden, and hazel.
Moving northward, the climate grows colder. The vast majority of Svealand and Norrland sits within the cool-temperate zone, which is marked by shorter growing seasons and long winters. In this region, the largest share of annual precipitation falls during the growing season. Coniferous trees dominate the cool-temperate zone, giving rise to its name: "the northern coniferous forest belt" (the taiga). Where the coniferous forest ends in the northernmost reaches of Europe, the tundra takes over, where trees struggle to survive due to permanently frozen ground (permafrost).
Precipitation in Sweden
In terms of precipitation, the largest part of Sweden can be classified as a humid region because annual precipitation exceeds total evaporation and plant transpiration. This water surplus is the reason behind Sweden's abundance of rivers and streams. However, annual humidity is not always an accurate gauge of water availability during the actual growing season. Because evaporation and transpiration peak during the summer, the water balance can periodically turn negative, creating a precipitation deficit. This scenario is most common in eastern and southern Sweden.
Sweden's Varied Temperature Climate
Due to its elongated north-south shape, Sweden’s temperature climate in the south differs drastically from that in the north. The overall picture is further shaped by the influence of large lakes, the surrounding seas, the Gulf Stream, and elevation. Annual mean temperature reveals relatively little about the practical climate; a location with hot summers and harsh winters can share the same annual mean temperature as a place with a much steadier temperature distribution throughout the year.
For vegetation, the temperature climate during the active growing season is what matters most. This is typically quantified as a temperature sum—the combined daily temperatures exceeding +5°C. This threshold is also what determines the exact length of the growing season.
Contact
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PersonJohan Stendahl, head of department and researcherBiogeochemistry of Forest Soils