An isobar is a line on a weather map that connects points of equal atmospheric pressure at a given moment, most often drawn on surface analyses at intervals of 4 millibars. Each isobar is labeled with the pressure value it represents, and the pattern of contours reveals highs, lows, ridges, and troughs at a glance.
How isobars are drawn
On a surface chart, station pressures are first reduced to sea level so that stations at different elevations can be compared fairly. Contours are then drawn through points of equal reduced pressure. TheWeatherPrediction summarizes an isobar simply as a contour of constant pressure, labeled by its pressure value.
Closed loops mark centers. A ring of decreasing pressure around a central minimum defines a low. A ring of increasing pressure around a central maximum defines a high. Elongated features without closed contours are troughs and ridges.
How forecasters read them
Spacing is the first thing to look at. NOAA JetStream describes the horizontal pressure difference between locations as the pressure gradient force, and the tighter the spacing between isobars, the stronger that force and the stronger the wind. Widely spaced isobars mean light wind. Closely packed isobars mean strong wind.
Direction comes next. Aloft, in the free atmosphere, wind flows nearly parallel to the isobars with lower pressure to its left in the Northern Hemisphere. Near the ground, friction slows the wind and tilts it roughly 20 to 30 degrees across the isobars toward lower pressure, which is what drives surface convergence into lows and divergence out of highs.
Where it misleads
An isobar map is a snapshot. A pattern with tightly packed isobars in the morning can relax by afternoon, and reading a single map without a time trend can lead to over-interpreting the wind or missing the deepening of a nearby low.
Pressure gradient alone also does not translate directly into surface wind speed. Terrain, stability, and boundary-layer decoupling at night can suppress the wind the isobar spacing implies. Use the pattern to identify where wind and convergence should be strongest, then check observations to confirm.