isobare

Isobare is a line on a weather map that connects locations sharing the same atmospheric pressure, typically the sea-level pressure. In English the standard term is isobar, while isobare is common in several languages such as German, French, and Italian. The pressure values are measured in hectopascals (hPa), with 1 hPa equal to 1 millibar.

On surface weather maps, isobares form patterns that reveal the large-scale pressure field. They usually appear as closed loops around high-pressure centers (anticyclones) and low-pressure centers (cyclones). The distance between adjacent isobares denotes the pressure gradient; tightly packed lines indicate strong winds, while widely spaced lines indicate lighter winds. Isobares are often labeled with the pressure value to aid interpretation.

Interpreting isobares helps meteorologists anticipate weather changes. Because winds tend to flow parallel to isobars due to the balance between pressure gradient and rotation (Coriolis effect) and friction near the surface, the arrangement of isobares implies wind direction around pressure systems. In the Northern Hemisphere, winds circulate clockwise around highs and counterclockwise around lows; in the Southern Hemisphere the reverse.

Isobare are used in conjunction with other map features such as isotherms and fronts to describe weather patterns, and they underpin routine surface weather charts used by meteorological services worldwide.