APPENDIX D$_{ }$ D.2.3.1 Wind Speed and Direction  D.2.3.2 Mixing Height  76 Appendix W to Part 51 –  Guideline on Air Quality Models , http://www.ecfr.gov/cgi-bin/text- idx?SID=e6a5b817b94abf58460f48c032d9a39c&node=40:2.0.1.1.2.23.11.5.37&rgn=div9.  AERMOD assumes that wind speed is constant from one direction for a given time period  being modeled, usually one hour. Wind speeds are usually measured by an anemometer at a  height of 20 feet. These measurements may or may not be corrected by the air quality analyst  to account for increasing wind speed with height.  For those time periods in which the wind speed is given as zero or “calm”, dispersion models  will generally assign a minimum wind speed. If a wind speed of zero is specified, AERMOD  computes an infinite concentration of the pollutant at the source, with no dispersion. In  reality, diffusion of pollutants into surrounding atmosphere would take place in calm  conditions.  Atmospheric stability is related to the turbulence of the atmosphere and is determined by a  combination of wind speed, cloud cover, and solar radiation.  In unstable atmospheric conditions, high turbulence and associated vertical mixing produce a  peak ground-level pollutant concentration near the emission source.  In stable atmospheric  conditions, a low level of vertical mixing results in low ground-level steady-state concentrations  near the source. In most cases, the most unstable atmospheric conditions occur during daylight  hours, with low wind speeds and high solar radiation. The most stable atmospheric conditions  occur at night, during times of low wind speeds and clear skies.  The presence of a stable layer above the mixing layer has the effect of restricting vertical  diffusion of pollutants. This “lidding” effect requires a modification for it to remain accurate  at distances greater than several kilometers downwind of the emission source. For applications  such as airports, however, where the pollutant of concern is not likely to be transported at  high concentration very far from the source, mixing depth effects on downwind pollutant  concentrations may be ignored without too much loss of accuracy.  77$_{ }$ The term “atmospheric mixing height” generally describes the height above ground level (AGL)  where most air pollutants are ge nerated and where atmospheric mixing occurs. Within the  atmosphere, this height (expressed in meters or feet AGL) is determined by an assortment of  factors including air temperature, humidity, solar radiation, wind speed, and topographic features  on the ground (i.e., valleys, mountains, vegetative cover, reflective and impervious surfaces,  water bodies, etc.). The atmospheric mixing height is dynamic moves up or down spatially and temporally throughout the day, season, and year with corresponding c hanges in  these abovementioned factors. The height of this mixing layer generally ranges between 1,000  and 6,000 feet AGL.  The EPA has established guidelines for the determination of atmospheric mixing heights used for  computing an emission inventory and for conducting atmospheric dispersion modeling. Section 5.2.2 ( Mixing Height Determination ) of the  Guideline on Air Quality Models 76 , the  following preamble discusses the application of mixing heights as it applies to aircraft emissions: 