Isobars

Isobars, in meteorology, are lines of equal atmospheric pressure. Barometric readings are taken at a large number of stations all over the earth; their average value at every station, during a certain interval, is carefully calculated; and lines are drawn on a map of the world, each line passing through those points where the pressure has been the same during the period specified. Study of these isobaric lines helps considerably in determining the cause of prevalent winds.