3.6 Recommendations

Recommendations for siting and exposure of in situ meteorological sensors in simple terrain are as follows: 

Sensors for wind speed and wind direction should be located over level, open terrain at a height of 10 m above ground level and at a distance at least ten times the height of nearby obstructions. For elevated releases, additional measurements should be made at stack top or 100 m, whichever is lower. Monitoring requirements for stacks 200 m and above should be determined in consultation with the appropriate EPA Regional Office.

Temperature sensors should be located at 2 m. Probe placement for temperature difference measurements depend on the application. For use in estimating surface layer stability, the measurement should be made between 20z₀ and 100z₀ ; the same recommendation applies to temperature difference measurements for use in estimating the P-G stability category using the solar radiation delta-T method. For use in estimating stable plume rise, temperature difference measurements should be made across the plume rise layer, a minimum separation of 50 m is recommended for this application. Temperature sensors should be shielded to protect them from thermal radiation and any significant heat sources or sinks. 

Pyranometers used for measuring incoming (solar) radiation should be located with an unrestricted view of the sky in all directions during all seasons. Sensor height is not critical for pyranometers; a tall platform or rooftop is an acceptable location. Net radiometers should be mounted about 1 m above ground level.

Specific recommendations applicable to siting and exposure of meteorological instruments in complex terrain are not possible. Generally, one should begin the process by conducting a screening analysis to determine, among other things, what terrain features are likely to be important; the screening analysis should also identify potential worse case meteorological conditions. This information should then be used to design a monitoring plan for the specific application.

Special siting considerations also apply to coastal and urban sites. Multiple sites, though often desirable, may not always be possible in these situations. In general, site selection for meteorological monitoring in support of regulatory modeling applications in coastal and urban locations should be conducted in consultation with the appropriate EPA Regional Office. 

If the recommendations in this section cannot be achieved, then alternate approaches

should be developed in consultation with the appropriate EPA Regional Office. Approval of site. selection for meteorological monitoring should be obtained from the permit granting authority prior to installation of any equipment.

3. SITING AND EXPOSURE 
 3.1 Representativeness 
     3.1.1 Objectives for Siting 
     3.1.2 Factors to Consider  
 3.2 Simple Terrain Locations 
     3.2.1 Speed and Wind Direction  
     3.2.2 Temperature, Temperature Difference, and Humidity
     3.2.3 Precipitation  
     3.2.4 Pressure 
      3.2.5 Radiation 
 3.3 Complex Terrain Locations  
     3.3.1 Wind Speed  
     3.3.2 Wind Direction 
     3.3.3 Temperature Difference 
  3.4 Coastal Locations
 3.5 Urban Locations 
 3.6 Recommendations