2 PRIMARY METEOROLOGICAL VARIABLES

This section provides general information on the instruments used for in situ measurements of wind speed, wind direction, temperature, temperature difference, humidity, precipitation, pressure, and solar radiation. These variables are considered primary in that they are generally measured directly. Derived variables, such as atmospheric stability, mixing height, and turbulence are discussed in Section 6. Remote sensing platforms for measurements of winds, temperature, and humidity are discussed in Section 9; these variables, when determined using remote sensing, are not measured directly, but are derived from other measurements.

The choice of an instrument for a particular application should be guided by the data quality objectives of the application; as a minimum, these objectives should include the accuracy and resolution of the data needed by the application - recommended data quality objectives for regulatory dispersion modeling applications are provided in Section 5.0. Other considerations which may compete with the data quality objectives include the cost of the instrument, the need for and cost of routine maintenance, and the competing needs of ruggedness and sensitivity. One should also note that the cost of a successful monitoring program does not end with the purchase of the sensors; depending on the instrument, additional costs may be incurred for signal conditioning and recording hardware. There are also the costs involved in siting, installation, and calibration of the equipment, as well as costs associated with the quality assurance and processing of the data.

The focus in the following is on those classes of instruments that are considered best suited for routine in situ monitoring programs, and which generally have had the widest use. Additional information and illustrations for the instruments described in this section may be found in references [2], [5], [6], [7], and [8].

2. PRIMARY METEOROLOGICAL VARIABLES 
 2.1 Wind Speed
      2.1.1 Cup Anemometers
      2.1.2 Vane-oriented and Fixed-mount Propeller Anemometers 
      2.1.3 Wind Speed Transducers 
  2.2 Wind Direction 
      2.2.1 Wind Vanes  
      2.2.2 U-V and UVW Systems  
      2.2.3 Wind Direction Transducers 
      2.2.4 Standard Deviation and Turbulence Data  
  2.3 Temperature and Temperature Difference 
      2.3.1 Classes of Temperature Sensors  
      2.3.2 Response Characteristics  
      2.3.3 Temperature Difference 
      2.3.4 Sources of Error 
  2.4 Humidity  
      2.4.1 Humidity Variables  
      2.4.2 Types of Instrumentation  
  2.5 Precipitation 
  2.6 Pressure  
  2.7 Radiation  
  2.8 Recommendations