Isobars (literally "iso" - equal, and bar "pressure") mean lines of equal pressure. Unfortunately, in meteorology, there are many different pressure settings and levels we could be talking about. Isobars on surface pressure charts refer to the mean sea level pressure as accurately as it can be calculated, which means using actual pressure lapse rates, not the hypothetical ones in ISAISA —International Standard Atmosphere. The proper term for pressure at mean sea level using actual lapse rates is QFFQFF —Static pressure at MSL calculated from QFE using actual temperature lapse rates - colloquially thought of as "for forecasters."
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What, if anything, do isobars join?