the sigma coordinate system allows sigma-surfaces to follow model terrain; where terrain is sharply sloped, so are the sigma surfaces. this allows for continuous fields, such as temperature, to be represented especially smoothly at the lowest layers in the model. further, with the exponential decaying nature of density within the atmosphere, sigma coordinates provide a greater vertical resolution (in terms of metres) near the surface. the sloping nature of the coordinate surfaces does require additional interpolation of the pressure gradient force, and the smoothing of terrain can often cause it to extend beyond the true boundaries of land.