The Complex Terrain Dispersion Model (CTDM) is a refined air quality model for use in stable and neutral conditions in complex terrain applications. Its use of meteorological input data and terrain information is different than current EPA models; considerable detail for both types of input data are required and are supplied by preprocessors specifically designed for CTDM. CTDM requires the parameterization of individual hill shapes using the terrain preprocessor and the association of each model receptor with a particular hill (except for receptors in flat terrain, which CTDM can also handle). A central feature of CTDM is its use of a critical dividing-streamline height (Hc) to separate the flow in the vicinity of a hill into two separate layers. Flow in the upper layer has sufficient kinetic energy to pass over the top of the hill while streamlines in the lower layer are constrained to flow in a horizontal plane around the hill.