Figure 1. Schematics depiction of the potential temperature field in the atmosphere. Left panel shows that in the actual state, potential temperature varies with latitude, with cold air at in the surface layers in the polar regions (indicated in purple). Right panel shows the reference or global equilibrium state of the atmosphere, obtained by allowing each layer to relax towards their equilibrium state while retaining their mass and potential temperature. The equilibrium state is horizontally uniform and therefore a function of height or pressure only. Reproduced from Tailleux (2013).