isobar or isobaric line, line drawn on a weather map through points of equal atmospheric pressure. Isobars are used to define cyclones (low-pressure regions) and anticyclones (high-pressure regions). Weather maps are designed to depict the horizontal pressure distribution across an area of land, but atmospheric pressure also varies vertically, i.e., with altitude. To eliminate any consideration of the vertical variations of pressure, the barometer readings at all stations are reduced to their corresponding sea-level pressures before the isobars are drawn.