Global warming or climate change has increased the frequency of extreme temperatures. This is reflected in the increase in the number of tropical days (days with temperatures above 30 °C); these are increasing every year. On these days in particular, there can be negative impairment of functions in the vineyard or direct damage to the leaves and berries of the vines. The potential danger is mainly caused by a hot period with very high temperatures after a cool period and very low humidity. Other causes are too radical defoliation in the grape zone or defoliation due to heavy hailstorms. The effects are burn damage to exposed berries or grape wilting after previous drying of the berry stalks. In the ripening phase, extreme temperatures have a significant influence on malic acid; temperatures of over 40 °Celsius lead to a drastic reduction in berry yields. Particularly sensitive are the varieties Chardonnay, Grüner Veltliner, Muskateller, Riesling, St. Laurent and Zweigelt. Regarding preventive measures against damage, see under Canopy Management.