Their climate & flora:
The extensive Rila mountains are subject to moderate continental and transitional continental climates. In general, the average annual temperature is moderate, although Mt. Musala is recorded as being the coldest place in Bulgaria. On its summit during the summer the temperature rises to only a few degrees above freezing. The climate gives rise to a very rich and diverse flora with plants typical for Central Europe, sub-Mediterranean areas and the Balkans. Several rare, endemic species grow in the Rila.
The lower parts of the Rila most typically are wooded with Hornbeam (Carpinus betulus) and Sessile Oak (Quercus petraea), and in the Struma valley Hungarian Oak (Q. frainetto), Oriental Hornbeam (Carpinus orientalis) Turkey Oak (Q. cerris) and White Oak (Q. pubescens) are widespread. At 1300 to 1600m asl., Beech (Fagus sylvatica) that in many places is mixed with Aspen (Populus tremula), Silver Birch (Betula pendula) and Silver Fir (Abies alba) is the dominant vegetation. Coniferous forests of Macedonian Pine (P. peuce), Norway Spruce (Picea abies), and Scots Pine (Pinus sylvestris) cover the mountain-sides between 1600 to 2000m asl., and above 2000m asl., the sub-alpine zone is comprised of peat pastures and stone ridged meadows containing flowers such as Alpine Bellflower (Campanula alpina), Dwarf Snowbell (Soldanella pusilla), Mountain Avens (Dryas octopetala), Mountain Thrift (Armeria maritima aipina), Spring Pasque Flower (Pulsatilla vernalis), and Gentiana and Primula spp., including endemic Primula deorum.
The western Rhodope have a transitional continental climate, and the Eastern Rhodope have a distinctly Mediterranean climate. January and February are the coldest, and July and August the warmest, months. Highest precipitation occurs during May and June in the western area, whereas this occurs in December in the eastern area although this area also experiences short, sharp summer electrical storms.
The lower Rhodope have typical sub-Mediterranean forests of Hungarian Oak (Quarks frighten) and White Oak (Q. pubescent) that give way to Sessile Oak (Q. petrel) and then, above 1000m asl., Beech (Fagus sylvatica) and also in many places Aspen (Populus tremula), Silver Birch (Betula pendula) and Silver Fir (Abies alba). Above this the coniferous forests are comprised mainly of Norway Spruce (Picea abies) and Scots Pine (Pinus sylvestris) and higher still there are lush meadows rich in Central European, Mediterranean and Balkan species. Of particular interest is the rock plant Haberlea rhodopensis that has clusters of small, pale blue-violet trumpet-shaped flowers.
The Pirin are more subject to the influence of Mediterranean air masses than most other mountains in Bulgaria. However, their extreme altitude, a third of the total area of the Pirin is above 1600m asl., has a significant effect on the climatic effects in the mountains. Both altitude and location significantly influence precipitation and, for example, the average annual total for Mt. Vihren is more than double that of the surrounding valley bottoms. Much of the winter precipitation falls as snow that lies in the mountains for 5 to 8 months with the heaviest snowfalls occurring in February and March providing the best winter sports conditions. November and December are usually the wettest months with August and September usually being the driest. The peripheries of the Pirin are substantially warmer than the interior areas and the warmest months are July and August. Visibility is usually excellent during summer and autumn, but during the winter and early spring dense cloud can shroud the tops of the peaks.
As a result of their location, geology and especially climate, the Pirin have a wide and diverse mixture of Central European, Alpine and Mediterranean flora species. The low valleys have woods mainly comprised of Hornbeam (Caryinus betulus) and Sessile Oak (Quercus petraea), but at the southwestern end the woods are predominately a mixture of Hungarian Oak (Q. frainetto), Oriental Hornbeam (C. orientalis), Turkey Oak (Q. cerris), White Oak (Q. pubescens), and some Sweet Chestnut (Castanea sativa). Between 1400 and 1800m asl., the extensive forests are predominately a mixture of Beech (Fagus sylvatica) and Silver Fir (Abies alba); between 1800 to 2000m asl., the coniferous forests are comprised of Macedonian Pine (P. peuce), Norway Spruce (Picea abies), and Scots Pine (Pinus sylvestris), and above 2000m asl., the forests are replaced first with Dwarf Pine (P. mugo) and Siberian Juniper (Juniperus sibirica), and then sub-alpine and alpine meadows containing many rare or endemic species including flowers such as Alpine Bellflower (Campanula alpina), Dwarf Snowbell (Soldanella pusilla), Least Primrose (Primula minima), Mountain Avens (Dryas octopetala), and Spring Gentian (Gentiana verna).