Constance is one of the best known wine growing regions in Germany and experiences several outdoor events during harvest time in autumn. Lake Constance is also considered to be a risky and challenging lake for water sports because of the danger of gusty winds which can whip up waves as the weather changes suddenly. The most dangerous wind is the föhn, a warm down-slope wind from the Alps, which spreads out across the water, especially through the Alpine Rhine Valley and can generate waves several metres high. Similarly dangerous for those unfamiliar with the area, are the sudden stormy gusts of wind during summer thunderstorms. They constantly claim victims from the water sports fraternity. During a thunderstorm in July 2006, waves reached heights of up to 3.50 metres. For these reasons, there is a storm warning system in all three neighbouring countries. For storm warning purposes, Lake Constance is divided into three warning regions (west, centre and east). Warnings can be issued for each region independently. A "high winds" warning will be issued when squalls are expected of between 25 and 33 knots or registering force 6 to 8 on the Beaufort scale. A gale warning announces the likelihood of gale-force winds, i.e. those at speeds as of 34 knots or more or force 8 on the Beaufort scale. In order to issue these warnings, orange-coloured flashing lights are installed around the lake, which flash at a frequency of 40 times per minute for high winds or 90 times per minute for gales. It can happen that, due to the differently regulated responsibilities and assessments, a gale warning is issued on the Swiss side of the Obersee, but not on the German or Austrian shores, and vice versa. Ships and ferries on Lake Constance indicate a gale warning by hoisting a Sturmballon ("storm ball") up the mast. A one-hundred year event is the freezing over of Lake Constance, when the Lower Lake, Lake Überlingen and the Upper Lake are completely frozen over so that people can safely cross the lake on foot. The three last so-called Seegfrörne events were in 1963, 1880 and 1830. Certain parts of the lake freeze over more frequently, mainly due to their shallow depth of water and shelter, as is the case, for example, of the so-called Markelfinger Winkel between the municipality of Markelfingen and the Mettnau peninsula. International borders The lake lies where the