Isobars that are drawn on surface weather maps follow a sequence of values that can be found by adding or subtracting 4 from 1000, then adding or subtracting another 4 from the resulting numbers, and so on. On the map above, the pressure inside the 1012-mb isobar is higher than the pressure outside that isobar. Complete the 1016-mb isobar and draw all other isobars in the sequence that spans the range of pressure values appearing on the map. When positioning isobars, assume a steady pressure change between neighboring stations. The isobar you are drawing will generally parallel the curves of its neighbors because horizontal changes in air pressure from place to place are usually gradual.
Isobars never stop or end within a data field, and they never fork, touch or cross one another.
Isobars cannot be skipped if their values fall within the range of air pressures reported on the map. Always label all isobars. The electronic portion of the Online Activity File printed from the Online Homepage .