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2.7 Radiation

Solar and/or net radiation data are used to:

  • determine atmospheric stability (Section 6.4.2),
  • for calculating various surface-layer parameters used in dispersion modeling (Section 6.6), 
  • for estimating convective (daytime) mixing heights, and 
  • for modeling photochemical reactions.

Solar radiation refers to the electromagnetic energy in the solar spectrum (0.10 to 4.0 Ám wavelength); the latter is commonly classified:

  • as ultraviolet (0.10 to 0.40 Ám), 
  • visible light (0.40 to 0.73 Ám), 
  • and near-infrared (0.73 to 4.0 Ám) radiation. 

Net radiation includes both solar radiation (also referred to as short-wave radiation) and terrestrial or long-wave radiation; the sign of the net radiation indicates the direction of the flux (a negative value indicates a net upward flux of energy).

Pyranometers are a class of instruments used for measuring energy fluxes in the solar spectrum. These instruments are configured to measure what is referred to as global solar radiation; i.e., direct plus diffuse (scattered) solar radiation incidence on a horizontal surface. The sensing element of the typical pyranometer is protected by a clear glass dome which both protects the sensing element, and functions as a filter preventing entry of energy outside the solar spectrum (i.e., long-wave radiation). The glass domes used on typical pyranometers are transparent to wavelengths in the range of 0.28 to 2.8 Ám. Filters can be used instead of the clear glass dome to measure radiation in different spectral intervals; e.g., ultraviolet radiation.

WMO specifications for several classes of pyranometers are given in Table 2-1 [9]. First class and secondary standard pyranometers typically employ a thermopile for the sensing element. The thermopile consists of a series of thermojunction pairs, an optically black primary junction, and an optically white reference junction (in some pyranometers, the reference thermojunction is embedded in the body of the instrument). The temperature difference between the primary and reference junctions which results when the pyranometer is operating generates an electrical potential proportional to the solar radiation. 

Second class pyranometers typically employ photo-cells for the sensing element. Though less costly than other types of pyranometers, the spectral response of the photovoltaic pyranometer is limited to the visible spectrum.

First class or second class pyranometers should normally be used for measuring global solar radiation, depending on the application. If the solar radiation data are to be used in procedures for estimating tability (Section 6.4) then second class (photovoltaic) pyranometers are acceptable. For most other applications, first class or secondary standard pyranometers should be used. Applications requiring ultraviolet (UV) radiation data should not employ photovoltaic measurements as these instruments are not sensitive to UV radiation. 

Table 2-1
Classification of Pyranometers [9]

Characteristic

UnitsSecondary
Standard
First
Class
Second
Class
Resolution>Wm-2>▒ 1>▒ 5>▒ 10
Stability>%FS*>▒ 1>▒ 2>▒ 5
Cosine Response>%>< ▒ 3>< ▒ 7>< ▒ 15
Azimuth Response>%>< ▒ 3>< ▒ 5>< ▒ 10
Temperature Response>%>▒ 1>▒ 2>▒ 5
Nonlinearity>%FS*>▒ 0.5>▒ 2>▒ 5
Spectral Sensitivity>%>▒ 2>▒ 5>▒ 10
Response Time>seconds>< 25>< 60>< 240
* Percent of full scale

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


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