Roman expansion across Western Europe brought Roman viticulture to the areas that would become some of the world's best-known winegrowing regions: the Spanish Rioja, the German Mosel, and the French Bordeaux, Burgundy and Rhône. Roman viticulturists were among the first to identify steep hillsides as one of the better locations to plant vines, because cool air runs downhill and gathers at the bottom of valleys. While some cool air is beneficial, too much can rob the vine of the heat it needs for photosynthesis, and in winter it increases the risk of frost.[17]