One of the mysteries of the English language finally explained.
A mountain ridge or other topographical feature which divides an airflow into distinct parts. Hence: a region in which the atmosphere behaves in a coherent way with respect to the dispersion of pollutants; the air supply of a given geographical or administrative region.
1920s; earliest use found in Geografiska Annaler. From air + shed, perhaps after German Luftscheide (although this is apparently first attested slightly later: 1926 or earlier), itself after Wasserscheide.
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