Isobars

the isolines of atmospheric pressure.

Isobaric charts are compiled most often for the perennial mean monthly pressure, the mean pressure of any period of time, or the pressure at a particular moment. To eliminate the influence of differences in the altitude of individual stations, the pressure measured at them is equated before the isobars are drawn to sea level or to another standard level according to a barometric formula (isobars on altitude weather charts).

atoms of different chemical elements but with the same mass number (A).

The nuclei of isobars contain the same number of nucleons but different numbers of protons (Z) and neutrons (N). For example, the atoms , and are three isobars with A = 10. The masses of isobars with the same A differ slightly because of differences in the binding energies of their nuclei. Isobars with the lowest masses are stable with respect to β-decay, while the heavier ones are unstable. Heavy isobars with an excess of protons are subject to positron β-decay or K-capture; those with an excess of neutrons, to electron β-decay. Mirror nuclei (found among light nuclei), a special case of isobars, are obtained by replacing protons by neutrons and neutrons by protons: for example, and or and .