Isobars
Isobars are the lines on a weather map that join places with the same air pressure. They are usually drawn every 4 millibars, and the closer together they are, the stronger the wind.
Easily confused with
How to read it
A Met Office teaching pack defines isobars as lines joining points of equal pressure, “similar to contours”, with pressures first corrected to mean sea level so that altitude does not affect them; it says they are normally drawn every 4 millibars. The Met Office’s guide to synoptic charts describes them as black lines, and says closely spaced isobars mean strong winds and widely spaced ones lighter winds. NOAA’s JetStream school says meteorologists draw them from the pressures on station plots.
The WMO’s manual on weather charts recommends an interval of 4 or 5 hPa (a hectopascal is a millibar) and says the 1,000 hPa isobar should always be among them. NOAA’s Weather Prediction Center draws isobars every 4 mb, and may add intermediate isobars every 1 or 2 mb where the pressure changes slowly.
Also searched as: curved lines on a weather map, isobar, lines on a weather map, millibar lines, pressure lines.
These pages describe what each symbol on a weather chart means. They are not forecasts or warnings: for those, the national weather service for the area is the source.
Look-alikes
Isobars look like the contour lines on a topographic map, and the Met Office makes the same comparison: both join points of equal value. On a weather map the values are pressures in millibars, such as 1008 or 1012, not heights. Closed rings of isobars surround the H and L pressure centres.
Read next
- Next in Weather map symbolsStation modelA station model, or station plot, is the cluster of numbers and symbols around a small circle on a weather map: the weather observed at one station. Temperature is at the upper left, pressure at the upper right, dew point at the lower left, and the circle shows the cloud cover.
- The same shape in Accessibility symbolsAccessible Icon (dynamic wheelchair symbol)A wheelchair symbol with the figure leaning forward and an arm swept back on the wheel, from the Accessible Icon Project. It marks the same accessible places as the International Symbol of Access; the project put it in the public domain.
More weather map symbols
The same shape elsewhere
Browse symbols drawn with circles or curves and waves.
Sources
- How to read synoptic weather charts, Learn about weather, Met Office, 1. Isobars and pressure patterns
- Understanding weather charts, Key Stage 4 lesson plan, Crown Copyright, Met Office (hosted by MetLink, Royal Meteorological Society), Part A: Isobars
- Manual on the Global Data-processing and Forecasting System, Appendix II-4: Graphical representation of data, analyses and forecasts, WMO-No. 485, Appendix II-4 (as circulated to IPET-DRMM-III, Beijing 2015), World Meteorological Organization, 4.1.2 Isobars
- Unified Surface Analysis Manual, Version 1, NOAA National Weather Service, Weather Prediction Center (with OPC, NHC and HFO), Chapter 1, Introduction
- JetStream Max: Surface Weather Plots, JetStream, the NWS online weather school, NOAA National Weather Service, Analyzing Weather Maps
- High and low pressure, Learn about weather, Met Office, High and low pressure
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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