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. Soils Fine, dark chernozem (black earth) soils, rich in loess and humus, occur over a considerable portion of the northern Danubian plateau. They are fertile, easy to work, and compare with the best soils in Europe. Away from the river, approaching the mountains, there is a broader area that is basically similar, but the subsoils are more porous and have allowed the humus and loess to leach downward from the surface. The resulting gray soil no longer rates among the finest, but it yields good crops in some areas and, where it is less satisfactory, the land is forested. The Thracian Plain has comparatively little of the finest soils, but it has much soil that is more than adequate to produce reasonably good crops. The best on the plain is locally called _smolnitsa_. It is basically a chernozem, but it is less fully matured and coarser than the darker variety along the Danube. The plain also features fairly extensive areas of good brown and brown forest soils. Meadow soils occur in large areas in the vicinity of Plovdiv. Some are irrigated and cultivated. Meadow and layered podzol (gray forest) soils occur in most of the higher elevations throughout the country. Intermediate elevations usually have brown forest soils, some of which are excellent. The Maritsa and Tundzha and the major rivers that flow into the Danube have wide valleys with alluvial soils. They may be coarse, but most of them are fertile, drain well, and are extensively cultivated. Vegetation Both the natural vegetation and the cultivated crops that have replaced it on all areas that could be put to agricultural use reflect the transitional climate of the country. North of the Stara Planina the original flora was a continuation of that on the Russian steppe. The steppe influence was greatest in the east, giving way to deciduous forests farther to the west.