Another important factor is the relief. The Bulgarian mountains and valleys act as barriers or channels for air masses, causing sharp contrasts in weather over relatively short distances. The Balkan Mountains form a barrier which effectively stops the cool air masses coming from the north and the warm masses from the south. The barrier effect of the Balkan Mountains is felt throughout the country: on the average, northern Bulgaria is about one degree cooler and receives about 192 mm more rain than lowlands of southern Bulgaria. The Rilo-Rhodope Massif bars the warm Mediterranean air masses and limits the Mediterranean influence to the southern valleys of the rivers Struma, Mesta, Maritsa and Tundzha, despite the close proximity of the Aegean Sea.