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.
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.