Considering its small area, Bulgaria has an unusually variable and complex climate. The country lies between the strongly contrasting continental and Mediterranean climatic zones. Bulgarian mountains and valleys act as barriers or channels for air masses, causing sharp contrasts in weather over relatively short distances. The continental zone is slightly larger, because continental air masses flow easily into the unobstructed Danubian Plain. The continental influence, stronger during the winter, produces abundant snowfall; the Mediterranean influence increases during the summer and produces hot, dry weather. The barrier effect of the Balkan Mountains is felt throughout the country: on the average, northern Bulgaria is about one degree cooler and receives about 192 more millimeters of rain than southern Bulgaria. Because the Black Sea is too small to be a primary influence over much of the country's weather, it only affects the immediate area along its coastline.
The Balkan Mountains are the southern boundary of the area in which continental air masses circulate freely. The Rhodope Mountains mark the northern limits of domination by Mediterranean weather systems. The area between, which includes the Thracian Plain, is influenced by a combination of the two systems, with the continental predominating. This combination produces a plains climate resembling that of the Corn Belt in the United States, with long summers and high humidity. The climate in this region is generally more severe than that of other parts of Europe in the same latitude. Because it is a transitional area, average temperatures and precipitation are erratic and may vary widely from year to year.
Average precipitation in Bulgaria is about 630 millimeters per year. Dobruja in the northeast, the Black Sea coastal area, and parts of the Thracian Plain usually receive less than 500 millimeters. The remainder of the Thracian Plain and the Danubian Plateau get less than the country average; the Thracian Plain is often subject to summer droughts. Higher elevations, which receive the most rainfall in the country, may average over 2,540 millimeters per year.
The many valley basins scattered through the uplands have temperature inversions resulting in stagnant air. Sofia is located in such a basin, but its elevation (about 530 meters) tends to moderate summer temperature and relieve oppressive high humidity. Sofia also is sheltered from the northern European winds by the mountains that surround its troughlike basin. Temperatures in Sofia average -2°C in January and about 21°C in August. The city's rainfall is near the country average, and the overall climate is pleasant.
The coastal climate is moderated by the Black Sea, but strong winds and violent local storms are frequent during the winter. Winters along the Danube River are bitterly cold, while sheltered valleys opening to the south along the Greek and Turkish borders may be as mild as areas along the Mediterranean or Aegean coasts.
Data as of June 1992