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.