An isobar is a line on a weather map connecting points of equal atmospheric pressure reduced to sea level. Isobars are the most fundamental tool of synoptic meteorology, revealing the structure of highs and lows, wind patterns, and frontal zones at a glance. They are conventionally drawn at 4 hPa intervals (1000, 1004, 1008, 1012, 1016…), with the standard reference value being 1013.25 hPa.
The spacing between isobars directly indicates wind strength: closely packed isobars mean a steep pressure gradient and strong winds, while widely spaced isobars indicate gentle breezes. Wind direction can be deduced using Buys Ballot's Law: stand with your back to the wind in the Northern Hemisphere and low pressure is to your left. In practice, surface friction causes wind to cross isobars at a slight angle (about 10–30°) towards lower pressure.
Isobars form closed loops around pressure centres — concentric circles around anticyclones and depressions. Elongated regions of low pressure between two highs are called troughs, while ridges are extensions of high pressure. The pattern of isobars on a surface analysis chart reveals the large-scale atmospheric flow, the position of fronts, and the likely weather for the coming hours. Complementary lines at upper levels (contours) serve a similar purpose on height charts such as the 500 hPa analysis.