In the Thracian Plain, south of the Stara Planina, the continental climate is modified somewhat by the influence of the Mediterranean Sea. Compared to the Danubian plateau, winters are less severe, and summers are longer and warmer. The number of frost-free days per year ranges from 198 to 206. A near-Mediterranean climate prevails in the valleys of the lower Struma, Mesta, and Maritsa rivers; in the Arda basin; and on the southern slopes of the Rodopi (or Rhodope Mountains) (see ch. 3). The mountains protect the inland valleys and basins from strong winds; summers there are hot, and winters are mild. Yet winters are not mild enough for the cultivation of Mediterranean crops, such as olives and citrus fruits. The Black Sea coast is warmer than the interior of the country in winter but cooler in summer; from 241 to 260 days in the year are frost free. Frequent gale storms and hot winds resembling the African sirocco, however, have an adverse influence on crops. Although annual rainfall is reported to average about forty inches on the higher mountain slopes and to reach seventy-five inches in the Rila mountain range, precipitation in most farming areas averages only twenty to twenty-five inches per year. Rainfall measures even less than twenty inches in the Plovdiv area and in the coastal districts of the Dobrudzha region in the northeast. Most of the rainfall occurs in the summer months, but the amount and timing of precipitation are often unfavorable for optimum crop growth. Drought conditions reached crisis proportions in 1958 and 1963 and were serious also in 1968. In 1972 most crops were adversely affected by a spring drought and excessive rains in the early fall; the grape crop was an almost total loss. Soils of superior and intermediate quality make up almost three-fourths of the country's surface. The Danubian plateau contains several grades of chernozem (black earth), which gradually give way to gray forest soils in the foothills of the Stara Planina. A degraded chernozem called _smolnitsa_, or pitch soil, predominates in the Thracian Plain, the Tundzha and Burgas lowlands, and the Sofia Basin. This central region is encircled at higher elevations by a belt of chestnut and brown forest soils. Similar chestnut soils are also found in the Strandzha upland, in the basins of the eastern Rodopi region, and in the Struma and Maritsa valleys. Brown forest soils and mountain meadow soils occur in the Stara Planina and in the Rila, Pirin, and western Rodopi. Alluvial soils, often of good quality, are found alongside the rivers, particularly the Danube and Maritsa, and also in several basins. LAND USE