Weather charts
What Are Isobars? Reading Pressure Charts for Sailing
Published · Last checked October 2026
Isobars are lines on a weather chart joining places with the same air pressure, usually drawn every 4 millibars. Their spacing shows wind strength: close isobars mean strong wind, wide spacing means light wind. In the northern hemisphere, wind blows roughly along the isobars, anticlockwise around lows and clockwise around highs, angled about 15 degrees inwards towards the low over the sea.
What the lines show
A surface pressure chart, or synoptic chart, shows pressure at sea level. Isobars connect points of equal pressure, like contour lines on a map connect points of equal height. They are usually drawn at 4 mb intervals, such as 1000, 1004 and 1008 mb.
- Low (L): closed isobars with pressure falling towards the centre. Unsettled, windy weather and fronts.
- High (H): closed isobars with pressure rising towards the centre. Settled, often lighter winds.
- Trough: a V-shaped extension of a low, often with showers or a front.
- Ridge: an extension of a high, usually a fine spell between lows.
- Col: a slack area between two highs and two lows. Light, variable winds and sometimes fog.
Isobar spacing and wind strength
The closer the isobars, the steeper the pressure gradient and the stronger the wind. This is the most useful thing a sailor can read from a chart. Forecasters use a geostrophic wind scale printed on the chart to convert spacing to wind speed, adjusted for latitude.
| Spacing between isobars | Approximate wind |
|---|---|
| Over 300 miles | Light, force 1 to 3 |
| About 150 to 300 miles | Moderate, force 4 to 5 |
| About 100 miles | Strong, force 6 to 7 |
| Under about 60 miles | Gale force or more |
Treat these figures as a rough guide only. Curvature matters too: wind around a tight low is a little less than the spacing suggests, and around a high a little more.
Wind direction from isobars
Above the friction layer, wind blows parallel to the isobars. At the sea surface, friction slows it and turns it inwards towards low pressure, by roughly 15 degrees over the sea and more over land. In the northern hemisphere, wind circulates anticlockwise around a low and clockwise around a high.
Using charts to plan
Look at a sequence of forecast charts, not just one. Watch how the lows move and deepen, where the isobars tighten and when fronts will reach you. If the isobars over your route tighten through the day, the wind will rise. Compare with the shipping forecast and with your own barometer readings. The weather forecasting lesson and advanced meteorology lesson go further.