Sentence:  Up on the ridges above the valley, way above the <WSD>fog line</WSD> at between 400 and 700m (1300 and 2300ft), are some great old Zinfandel vineyards, their origins dating back to the sites that were first planted by Italian immigrants back in the closing decade of the 19th century. 
Meaning of the word marked with <WSD> tags:  A division in (altitude) above which (fog) appears.