Definition: Isobars are lines of equal atmospheric pressure drawn on a meteorological map. Each line passes through a pressure of a given value, provided certain rules are followed. The rules for drawing isobars are-

1. Isobar lines may never cross or touch.
2. Isobar lines may only pass through pressures of 1000 + or - 4. In other words, allowable lines are 992, 996, 1000, 1004, 1008, and so on.
3. The atmospheric pressure is given in millibars (mb). One millibar = 0.02953 inches of mercury.

1. Pressure lines are usually corrected for sea level so any differences in pressure due to altitude are ignored.

The picture to the upper right shows an advanced weather map with isobar lines drawn on it. Notice that it is easy to locate high and low pressure zones as a result of the lines on the maps. Also remember that winds flow from high to low areas, so this gives meteorologists a chance to predict local wind patterns as well.
Try drawing your own weather maps at Jetstream - The Online Meteorology School.