The line of intersection of an atmospheric isopycnic surface with some other surface, for instance, a surface of constant elevation or pressure. An isopycnic surface is a surface in which the density of the air is constant. Since specific volume is the reciprocal of density, isosteric surfaces coincide with isopycnic surfaces. On a surface of constant pressure, isopycnics coincide with isotherms, because on such a surface, density is a function solely of temperature. On a constant-pressure surface, isopycnics lie close together when the field is strongly baroclinic and are absent when the field is barotropic.