A Budyko–Sellers model is an energy-balance model in which the annual mean temperature depends on , the latitude, the hemispheres being symmetric. At each latitude
where is the annual insolation of the latitude divided by its global mean, the divergence of the poleward heat transport, and the albedo takes its ice value poleward of the ice line , where falls below a critical temperature . Budyko wrote , a relaxation towards the global mean ; in the diffusive form, .