What does it mean when isobars are close together?
Strong wind. Close isobars mean a steep pressure gradient; widely spaced isobars mean light wind.
Isobars join places with the same pressure, usually every 4 hPa on UK charts. The closer they are, the stronger the wind. A geostrophic wind scale on the chart turns the spacing into an approximate speed.
- Northern hemisphere: wind goes anticlockwise round a low and clockwise round a high.
- Friction turns the surface wind in towards the low, roughly 10–15° over the sea and about 30° over land.
- Buys Ballot's law: back to the wind, low pressure is on your left (northern hemisphere).
Source: Standard RYA meteorology.
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