This creates a continental climate with marked diurnal temperature variations that include a dramatic drop in nighttime temperatures. Along with the moderating influence of the Danube river, the Wachau has a macroclimate which includes constant air and heat circulation, allowing for sugars and phenolic compounds to build up during the day but acids and aromas to be preserved by the cooler nights. This creates white wines that tend to be high in extract and acidity. Irrigation is often necessary during the peak months of the growing season when yearly rain totals fall below . The vineyard soils of the Wachau are varied but consist primarily of rock outcrops with occasional layers of loess. The geography of the region is characterized by steep, rocky river banks (as steep as those found in the Mosel and Côte-Rôtie) that have had vineyards terraced into the hillsides. Higher up on the hills, the soils are rich in iron deposits and contain mixtures of gneiss, granite and slate. Closer to the river and in the flatter plains areas that dot the region, the soil is more alluvial with loess, sand and gravel. The heart of the Wachau district is the "Vinea Wachau Nobilis Districtus" which includes a strip of vineyard land that stretches from the western extremity of Schwallenbach along the Danube through Spitz, Weissenkirchen, Dürnstein, Loiben and finally to Mautern. There are over 900 named vineyard sites in Wachau (known as "Rieden"). Among the sites that may appear on Wachau wine labels those of the "Loibenberg, Kellerberg, Achleiten, Klaus" and 'Singerriedel" are the most noted. There are over of vineyards in the Wachau with Grüner Veltliner being the most widely planted. The area is also well known for its Riesling that carry distinctive trademarks of their "terroir" in aroma and flavor profile. Other varieties grown in the Wachau include Chardonnay (sometimes called "Feinburgunder"), Neuburger, Gelber Muskateller, Pinot blanc, Traminer and Sauvignon blanc. Riesling is often planted on the most ideal vineyard location along the steep gneiss hillsides near the river while Grüner Veltliners seems to thrive on the loess and sand of the lower banks.