For so small an area, the climate varies widely and is unusually complex. Depending upon the depth to which they study the area, climatologists list six or more climatic subzones. The country lies on the line of transition between the strongly contrasting Eastern European continental and the Mediterranean climatic zones, and its mountains and valleys are local factors that act as barriers or channels to the air masses, contributing to sharp contrasts in weather over relatively short distances. The Black Sea, although too small to be a primary influence over much of the country's weather, also affects the immediate area along its coastline.
In general, continental systems prevail in the north. They are characterized by hot summers, cold winters, and precipitation well distributed throughout the year, a major portion of it in early summer thunderstorms. The Mediterranean climate that is influential most of the time in the south has mild, damp winters but hot, dry, rain-free summers. The Stara Planina marks the lower limits of the area in which continental air masses circulate freely in typical circumstances. In the area between them and the Danube River there is an extension of the climate that is common to east-central Europe and adjoining regions of the Soviet Union.
In the same fashion, the Rodopi marks the northern limits of domination by Mediterranean weather systems. The southern slopes of these mountains are sufficiently mild to merit the region's being called the Green Greece or Bulgarian California.
The area in between, which includes the Thracian Plain, is influenced by both types of climate, but more of the time by continental systems. The result is a plains climate resembling that of the corn belt in the United States, which is characterized by long summers and high humidity. The climate is generally more severe than that of Spain and the portions of Italy, France, and Soviet Georgia that are in the same latitude. Because it is a transitional area and the Mediterranean systems may prevail for most of some seasons or retreat from the scene altogether in other seasons, average temperatures and precipitation are erratic and may vary widely from year to year.
Precipitation over the country averages about twenty-five inches a year and, when it is distributed normally throughout the seasons, it is satisfactory for most agricultural crops. Dobrudzha, in the northeast, the Black Sea coastal area, and parts of the Thracian Plain usually receive less than twenty inches. The remainder of the Thracian Plain and the Danubian plateau get less than the country average. Higher elevations are the most generously watered, in some places receiving forty inches or more.
Although a low figure of 7.6 inches was recorded in Dobrudzha for one year and the normal precipitation is marginal, both Dobrudzha and the Danubian plateau are in the continental climate zone and usually receive most of their rainfall during crop-growing seasons. The Thracian Plain, however, has frequent seasons when it is under Mediterranean influences and, when this is the case, it may experience prolonged summer droughts. Irrigation is, therefore, necessary for dependable agricultural production.
A few sheltered pockets in the higher mountains may remain covered with snow all year, and much of the other higher land remains white well into springtime. Lower elevations are snow covered an average of twenty-five to thirty days a year. Average cloudiness is about 55 percent, and average relative humidity is as high as 70 to 75 percent.
The many valley basins throughout the uplands frequently have temperature inversions resulting in stagnant air. The Sofia area, for example, is occasionally troubled by smog. The city's elevation of about 1,800 feet, however, tends to moderate summer temperatures and to relieve the oppressive quality of the high humidity. It is also sheltered from the northern European winds by the mountains that ring the basin. Its temperatures in January average about 29°F, and in August they average about 70°F. Its rainfall is near the country average, and the overall result of the several contributing features is a rather unexpectedly pleasant climate.
The climate of the coast is moderated by the Black Sea, but there are many windy days and violent local storms during the winter. The area along the Danube River experiences bitterly cold winters, and sheltered valleys opening to the south along the Greek and Turkish borders may, in contrast, be as mild as though they were on the Mediterranean or Aegean coasts. The so-called Black Wind, a local phenomenon similar to the African sirocco, consists of hard-blowing, hot, very dry air and wreaks havoc on crops. It gets its name from the quantities of dust it carries, which often darken the skies.
Regions in the Rodopi and the higher elevations around Sofia feature sun and snow in a pleasant combination for about four months a year. Several places have good and reasonably dependable skiing and are being developed into holiday resorts.