Milan sits on the heavily light-polluted Po Valley, one of the brightest regions in Europe, with Bortle 8 skies across the city. Relief comes from the Alps to the north and the Apennines to the south: the foothills reach Bortle 4 within an hour, and higher Alpine valleys offer Bortle 3 darkness with excellent altitude transparency. The Po Valley's summer humidity and winter fog (the famous nebbia) limit clear nights, so observers watch for the dry, clear days that follow Alpine high pressure. At 45.5°N, Milan has classic northern-hemisphere skies.