Varying climatic influences in the territory of the Republic of Srpska are the consequence of natural elements and the general laws of the circulation of air masses over wide expanses. The northern peri-Pannonian part belongs to the moderate continental climate belt, but the Pannonian-steppe climatic influence from the north is also felt. The summers are hot and the winters are moderately cold with the average annual temperature above 10°C.
Precipitation is rather uniformly distributed and the largest amount falls when agricultural crops need it most (May to June). As a rule, rainfall decreases from the north (1,500 mm) towards the east (700 mm) due to the influence of west air streams.
Mountain and mountain-valley (sub-mountain) variations are predominant climatic influences in the Republic of Srpska. Large mountain massifs have the mountain climate which is characteristic of cold and short summers and long and snowy winters. Snow fall is abundant and snow remains for a long time exceeding 1,200 mm. Contrary to the mountain climate in these regions, hilly areas, ravines and valleys have somewhat milder climate - the mountain-valley climate. The major characteristics of this climate are moderately warm summers and pretty cold winters with the average annual temperature below 10°C and the precipitation of 700-1000 mm.
The southern part of the Republic of Srpska, the territory of Herzegovina (Humine), has a modified variant of the Adriatic climate, while the Rudine area is characteristic of a transitional variant between the Humine climate and the mountain climate. The Humine climate is characteristic of a feeble influence of the sea, so the summers are very warm and the winters are pretty mild. The average annual temperature ranges from 11°C to 14°C. The distribution of precipitation is unfavourable since most rain falls in autumn and winter and least in summer when there are short periods of draught. Rain prevails over snow. Contrary to such features of the climate of the Herzegovina Humine, the climate of Rudine is mostly characteristic of lower summer temperatures and considerable snow fall in winter.
From the aspect of hydrology, the area of the Republic of Srpska can be regarded as relatively rich in surface and ground waters. All the main river streams belong to the Black Sea watershed. The major river stream in the water-shed is the Sava River which lies at low altitude, so all larger water courses - the Una with the Sana, the Vrbas, the Ukrina, the Bosna and the Drina flow into it. All of these rivers have complex and steep river basins with considerable water power. Especially important in this sense is the Drina river which has the most developed hydrology system and enormous water power that has not been completely evaluated yet. The other river courses are also important for the economy although only their lower and middle sections belong to the Republic of Srpska. The exception is the River
Ukrina which has a small course and flows through the hills in between the lower courses of the rivers Vrbas and Bosna.
From the aspect of hydrography the river network of the Republic of Srpska is not complete unless the largest and the most important river course in the Serb Herzegovina is taken into account - the Trebisnica whose water power has been completely utilized. This river as well as almost the whole underground karst hydrographic network belongs to the Adriatic Sea watershed.
The rivers in the Republic of Srpska, apart from their water power significantly contribute to tourism. This primarily relates to mountain river courses that are very clear and abound in fish.
Apart from being rich in rivers, the Republic of Srpska has numerous springs and wells primarily originating in cracks and contacts; valuable thermo-mineral springs lie in numerous faults (e.g. the Sava trench) and have enormous importance for the spa tourism. Finally, there are water storages and natural lakes, mostly in cirques.
The vegetation in the Republic of Srpska is very diversified, which is largely due to the climate. Particularly significant is the forest complex and the most widespread is the clima-zonal vegetation of oak forests. In the north, along the banks of the Sava river and its large tributaries lies a belt of English oak which is in places intersected by forests of sessile-flowered oak and simple hornbeam. This belt is the continuation of the forest belt in Slavonia known as the "Slavonian oak woods". Further south, on the hillocks and raised land, there is a forest community of sessile-flowered oak and simple hornbeam with beech woods of the submountain belt.
Since this flora-geographical area, Pannonian and submountain, is also most densely populated with farm population, forest communities spread on small areas only. Small localities of sessile-flowered oak and simple hornbeam can be found in the Neretva river valley in Serb Herzegovina.
The narrow belt of Gornje Podrinje and the lowland of Herzegovina are characteristic of climazonal vegetation of pubescent oak and European ash, and along the middle course of the Drina river and in the vicinity of Srbinje and Visegrad there is a narrow belt of pubescent oak and European Turkey oak.
A large part of the territory of the Republic of Srpska is under beech forest with mixed forest communities of beech, fir and spruce on high mountains. Besides beech, there are forests of maple, elm, European ash and other species.
This belt of quality trees is significant economically, but unfortunately the forests are excessively exploited and insufficiently regenerated. The beech belt extends in the subDinaric and Dinaric flora-geographical area. There are more of these forests in the eastern parts than in the western parts of the Republic of Srpska.
In forest grown areas, particularly on hills and mountains, there are large expanses of grass - meadows and pastures which are particularly significant for cattle breeding. The vegetation in the Republic of Srpska is quite interesting because of the variety of endemic species, their main representative being the Pancic spruce in the Drina River valley (the virgin forest of Perucica near Srbinje).
