Isobars are lines on a weather map connecting points with equal atmospheric pressure. Forecasters use them to display pressure patterns and estimate the strength and direction of surface winds.
Closely spaced isobars indicate a steep horizontal pressure gradient. Because air accelerates when pressure changes rapidly across a short distance, tightly packed lines usually signal stronger winds. Widely spaced isobars generally indicate a weaker pressure gradient and lighter winds, although friction and local terrain also matter.
Closed isobars often surround areas of high or low pressure. In the Northern Hemisphere, surface winds circulate clockwise around highs and counterclockwise around lows, with air crossing the lines slightly because of friction. The pattern reverses in the Southern Hemisphere.
Isobars are different from isotherms, which connect equal temperatures, and isohyets, which connect equal rainfall amounts. The prefix “iso-” means equal, while “bar” refers to pressure.