For the purposes of this guidance, the term "complex terrain" is intended to mean any site where terrain effects on meteorological measurements may be significant. Terrain effects include aerodynamic wakes, density-driven slope flows, channeling, and flow accelerations over the crest of terrain features, etc.; these flows primarily affect wind speed and wind direction measurements, however, temperature and humidity measurements may also be affected.

The definition of significance depends on the application; for regulatory dispersion modeling applications, significance is determined by comparing stack-top height and/or an estimated plume height to terrain height -

Terrain which is below stack-top is classified as simple terrain, terrain between stack-top height and plume height is classified as intermediate terrain, and terrain which is above plume height is classified as complex terrain.

The ideal arrangement in complex terrain involves siting a tall tower between the source and the terrain feature of concern. The tower should be tall enough to provide measurements at plume level.

Recommended steps in the siting process are as follows:

Define the variables that are needed for a particular application.

Develop as much information as possible to define what terrain influences are likely to be important. This should include examination of topographic maps of the area with terrain above physical stack height outlined. Preliminary estimates of plume rise should be made to determine a range of expected plume heights. If any site specific meteorological data are available, they should be analyzed to see what can be learned about the specific terrain effects on air flow patterns. An evaluation by a meteorologist based on a site visit would also be desirable.

Examine alternative measurement locations and techniques for required variables. Advantages and disadvantages of each technique/location should be considered, utilizing as a starting point the discussions presented above and elsewhere in this document.

Optimize network design by balancing advantages and disadvantages. It is particularly important in complex terrain to consider the end use of each variable separately.