Isobar

Also called pressure line, isobar line, millibar line

Weather-map symbol. A chart convention, not a forecast icon.

Isobar meaning

An isobar is a line of equal sea-level pressure on a weather map. It is not a front, and it is not the station’s three-digit pressure code.

At a glance

System: Synoptic chart convention
Used for: Equal pressure
Not the same as: A front, a trough, or a station pressure code

A line joining equal pressure. The number is the pressure. The line is not a front.

The name. A line of equal pressure. Not a front, and not the three-digit pressure code at a station.

Download drawing

Where it comes from

JetStream builds surface maps from station plots, then draws isobars through places with the same pressure. The station’s own pressure is the coded number, such as 103 for 1010.3 millibars on the Weather Prediction Center’s scheme. The isobar is the line that generalizes those numbers. Fronts may cross the pattern. They are not the isobars. This drawing is a single curve, unlabeled, so it cannot be mistaken for a row of triangles.

The line drawing is original to this cabinet. It is not an agency logo, a vendor icon, or the emoji for the same idea.

Weather

What it means

Pressure is the same all along the line. Lines packed close together mean the pressure changes quickly across them, which is a windier field. The barb, not the isobar, is the measured wind. A high and a low are centers the lines enclose. Reading an isobar as a rainfall boundary confuses it with a front or with a painted precipitation area.

Questions

What is an isobar?

A line of equal pressure, drawn from station reports. It is not a cold front and not a forecast icon.

Do close isobars mean rain?

They mean pressure changes quickly in a short distance, which usually means stronger wind. Rain is a different symbol.

Often confusedCold frontA front has triangles or half-moons and marks an air-mass boundary. An isobar is only equal pressure.

Often confusedPressure tendencyTendency is the three-hour change at one station. An isobar is a line across the map at one analysis time.

Near this

Sources

NOAA JetStream, surface weather maps

Named when the entry depends on a text, a date, or a standard. Catalog numbers are not invented.

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symbol.wiki, edited as Sigilla. Last reviewed 18 October 2026. A reading of public tradition, not a ruling.