Austria had signed a 1955 treaty re-establishing the country as a democratic and independent nation and subsequently declared its perpetual neutrality. In the 1960s, the city became the shooting and setting of the family musical film The Sound of Music. On 27 January 2006, the 250th anniversary of the birth of Wolfgang Amadeus Mozart, all 35 churches of Salzburg rang their bells after 8:00 p.m. (local time) to celebrate the occasion. Major celebrations took place throughout the year. As of 2017 Salzburg had a GDP per capita of €46,100, which was greater than the average for Austria and most European countries. Geography Salzburg is on the banks of the River Salzach, at the northern boundary of the Alps. The mountains to Salzburg's south contrast with the rolling plains to the north. The closest alpine peak, the 1,972‑metre-high Untersberg, is less than from the city center. The Altstadt, or "old town", is dominated by its baroque towers and churches and the massive Hohensalzburg Fortress. This area is flanked by two smaller hills, the Mönchsberg and Kapuzinerberg, which offer green relief within the city. Salzburg is approximately east of Munich, northwest of Ljubljana, Slovenia, and west of Vienna. Salzburg has about the same latitude as Seattle. Due to its proximity to the Austrian-German border, the greater Salzburg urban area has sometimes been regarded as, unofficially, including contiguous parts of Germany, including Freilassing (until 1923 known as Salzburghofen), Ainring and Piding. Climate Salzburg is part of the temperate zone. The Köppen climate classification specifies the climate as a humid continental climate (Dfb). However, with the isotherm for the coldest month, Salzburg can be classified as having four-season oceanic climate with significant temperature differences between seasons. Due to the location at the northern rim of the Alps, the amount of precipitation is comparatively high, mainly in the summer months. The specific drizzle is called Schnürlregen in the local dialect. In winter and spring, pronounced foehn winds regularly occur. Population Salzburg's official population significantly increased in 1935 when the city absorbed adjacent municipalities. After World War II, numerous refugees found a new home in the city. New residential space was constructed for American soldiers of the postwar occupation and could be used for refugees when they left. Around 1950, Salzburg passed the mark of 100,000 citizens, and in 2016, it reached the mark of 150000 citizens. Salzburg is home to large German, Bosnian, Serbian, and