IFS uses a hybrid sigma coordinate vertical coordinate, η, in which the lowest layers of the model are pure so-called 'sigma' levels, whilst the topmost model levels are pure pressure levels. The hybrid scheme provides for a smooth transition between the two. Pure sigma levels follow the surface topography and ensure no levels intersect the surface. A characteristic of the hybrid scheme is that the spacing between levels is less over steep orography than at the surface.
The vertical coordinate, η, is monotonic function of the pressure, p, and also depends on the surface pressure, ps, such that: