1911 Encyclopædia Britannica/Isobar
ISOBAR (from Gr. ἴσος, equal, and βάρος, weight), a line upon a meteorological map or pressure chart connecting points where the atmospheric pressure is the same at sea-level, or upon the earth’s surface. A general pressure map will indicate, by these lines, the average pressure for any month or season over large areas. The daily weather charts for more confined regions indicate the presence of a cyclonic or anticyclonic system by means of lines, which connect all places having the same barometric pressure at the same time. It is to be noted that isobaric lines are the intersections of inclined isobaric surfaces with the surface of the earth.