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isobar A line connecting points having the same atmospheric pressure reduced to a common datum, usually sea level. In other words, the phrase, for example, "along the 1020 isobar" would imply the a curved line on the surface of the earth having a constant pressure of 1020 millibars. Isobars are generally labeled every 4 millibars with just the last two digits of the pressure. A 1024 isobar would be labeled 24, see G139. In rare cases, we see a US surface analysis map with a few isolated intermediate isobars shown as dashed lines, which then makes the spacing 2 mb in those regions. See also millibar. Wind speed as a function of isobar spacing can be determined with the wind computer shown in G232. An exception to these notes is the 500-mb map where the reference plane is aloft and the lines shown are not actually isobars at all. See G254 and G180 for wind flow around isobars and G139 for a more complex example. A quick reminder of these conventions is in the Symbols Book accessible with G306. See also pressure gradient.

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