Considering its relatively small area, Bulgaria has variable and complex climate. The country occupies the southernmost part of the continental climatic zone, with small areas in the south falling within the Mediterranean climatic zone. The continental zone is predominant, because continental air masses flow easily into the unobstructed Danubian Plain. The continental influence, stronger during the winter, produces abundant snowfall; the Mediterranean influence increases during the second half of summer and produces hot and dry weather. Bulgaria is subdivided into five climatic zones: continental zone (Danubian Plain, Pre-Balkan and the higher valleys of the Transitional geomorphological region); transitional zone (Upper Thracian Plain, most of the Struma and Mesta valleys, the lower Sub-Balkan valleys); continental-Mediterranean zone (the southernmost areas of the Struma and Mesta valleys, the eastern Rhodope Mountains, Sakar and Strandzha); Black Sea zone along the coastline with an average length of 30–40 km inland; and alpine zone in the mountains above 1000 m altitude (central Balkan Mountains, Rila, Pirin, Vitosha, western Rhodope Mountains, etc.).