DEFINITION:

A line on a weather map that connects regions that have constant pressure.

SURFACE MAPS:

Isobars depict the state of the pressure field at the surface. The patterns give forecasters information to predict what the current state of atmosphere is and how it will change in the near future. For example, closed loops of isobars represent either regions of high or low pressure. Low pressure systems are usually associated with inclement weather conditions while high pressure systems tend to have clear skies. Winds rotate counter clockwise around low pressure systems and clockwise around high pressure systems in the Northern Hemisphere. The closer the isobars are to each other the stronger the winds will be due to the increased pressure gradient forces acting on the local parcels of air.

UPPER LEVEL MAPS:

Isobars in the middle atmosphere (i.e. 500 mb maps) outline the state of the pressure field above the Earth's surface. The patterns give forecasters information how the weather will evolve over time. Upper level wind patterns that flow parallel to the isobars act as a 'steering flow' for surface systems. Forecasters also can gain insight to whether a system will strengthen or weaken over time.