Although Switzerland is on a particularly suitable latitude for wine production, between 45 and 47 degrees, a high proportion of the country is simply too high. However, the country’s lakes and the föhn, a local wind which warms up sizeable portions of the south of the country, particularly Graubünden in the upper Rhine Valley, enable full grape ripening to take place in many valleys and on lakesides. And in the Valais in the south west, the upper Rhône Valley, sunshine is so dependable (an average of more than 2,000 hours a year, rising sometimes to more than 2,500 hours) that vineyards can be as high as 750 m/2,460 ft, and some, such as Visperterminen, reach 1,150 m above sea level. The Valais is sheltered by the alps and, like south east Switzerland, benefits particularly from the föhn, but it can be dry and irrigation with mountain water is sometimes necessary. Most Swiss wine regions have an annual rainfall of between 500 and 1,800 mm/19.5–70 in a year, the wettest region being Ticino, which suffers violent but short (hail) storms and is also the hottest with average July temperatures of more than 21 °C/70 °F. Elsewhere, average July temperatures are between 17.5 and 20 °C, there is good day–night temperature variability, and winter temperatures in the vineyards rarely fall below danger level for vines. The Valais is most at risk.