Climate is the long-term sum of temperature and precipitation. Regional climatic differences in Sweden are normally illustrated through division of the country into climatic zones.
The southernmost parts of the country are situated in the warm-temperate zone in which summers are relatively warm and long, where winters are not too severe. Deciduous species including oak, beech, elm, lime, and hazel are common in the forests. Towards the north the climate becomes harsher. Most of the country (north of the latitude of Stockholm) is in the cold-temperate zone, which is characterised by a relatively short growing season and long winters. Here the main part of the annual precipitation falls during the growing season. Coniferous species dominate forests, thereby the name 'northern conifer belt'. In the northernmost parts of Europe, at the border of the conifer belt, the permafrost zone begins and tundra has developed. On such frozen ground trees have difficulties to survive.
The main part of Sweden can concerning precipitation be considered as belonging to a humid region since the annual precipitation is larger than the annual sum of evaporation and transpiration, resulting in richness in lakes and watercourses. Annual humidity is, though, not always an ideal measure of water availability during the growing season. Much water is evaporated and transpired during summer and the water balance may periodically be negative, thus, resulting in a water deficit. Such a situation is common mainly in eastern and southern Sweden. Maps showing humidity and precipitation during the growing season give a more correct picture of the water availability.
Sweden is the in its main direction north-south a long country. Therefore the temperature climate differs considerably between the northern and southern parts. The general pattern with decreasing temperatures towards the north is modified through the influence from the large lakes, the sea, the Gulf Stream and altitude. The mean annual temperature does not really reflect the climate as such. Thus, the mean annual at a place with warm summers and cold winters could be similar at another place with temperature distributed more evenly around the year.
For vegetation the temperature climate during the growing season is very important. A useful index is given by the temperature sum which is the accumulated daily average temperature above 5ºC. This temperature limit is also used to define the length of the growing season.