Isobar

An isobar is a line on a synoptic weather chart joining points of equal atmospheric pressure, drawn at a fixed interval, commonly every 4 hectopascals, and reduced to mean sea level. The spacing gives the pressure gradient and so the wind: closely packed isobars mean a steep gradient and strong wind, widely spaced ones a slack gradient and light wind. Buys Ballot’s law places the low: in the northern hemisphere, stand back to the wind and the low pressure is on the left. Wind blows roughly along the isobars, the geostrophic and gradient balance, backed a little toward low pressure by surface friction. Reading isobars is the core skill of using a weather chart for passage planning.