## 4 Simulation Model Input Parameters
### Atmospheric Conditions  
Authors have previously developed a model for the calculation of spectral transmissivity of an atmospheric transfer path [18]. Using the HITRAN database, spectral absorption lines were previously calculated for the air of representative atmospheric composition at reference atmospheric conditions, permitting further conversion to any desired conditions. Four atmospheric parameters were recognized to have an influence on radiation thermometer measurement, namely air temperature, relative humidity, transfer path distance and atmospheric pressure. The effects of atmospheric temperature and relative humidity are simulated in Figure 7. As evident from the figure, the influence of unit temperature on transmissivity is more than twice greater of the influence of unit relative humidity.  
The best obtainable measurement capabilities of temperature and relative humidity range below 1 mK and 1% RH; however, this uncertainty does not correspond to the instability and non-homogeneity of climatic parameters within specific environments. Multiple uncertainty cases are thus considered in Table 2 in ascending uncertainty ranges.  
The reported best achievable measurement uncertainties of influence parameters (listed in column 2) and their statistical distributions (listed in column 3) are believed to be the best-case uncertainties in standard practices of climate parameter measurements. The best achievable measurement uncertainties of these input parameters have been assessed by the World Meteorological Organization's (WMO) Expert Team on Surface Technology and  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & **Best Achievable** & **Assessed** & **Laboratory Calibration Un-** \\  & **Uncertainty of** & **Practical** & **certainty Without Atmospheric Compensation** \\ \hline Air temperature & 0.2 K & 1–2 K & 4 K (uniform) \\ Relative humidity & 1\% RH & 5–10\% RH & 20\% RH (uniform) \\ Atmospheric pressure & 0.15 hPa & 10–20 hPa & 20 hPa (uniform) \\ Transfer path distance & 0.05 m & 0.05–1 m & 0.1 m \\ \hline \hline \end{tabular}
\end{table}
Table 2: Best achievable measurement uncertainties for influence parameters in expanded form (\(k=2\)). Unless otherwise noted, normal statistical distribution applies.  
Figure 7: Spectral transmissivity of air at 1013.25 hPa and 1 m distance (with spectral smoothing applied).  
Measurement Techniques (2004) in Anne 1.A of the Guide to Instruments and Methods of Observation [19].  
While these uncertainties are regarded as the best achievable uncertainties of measuring climatic conditions, we expect higher practical uncertainties (listed in column 4) when radiation thermometers (RTs) are being used in the field and when environmental parameters are measured with higher uncertainties and less frequently adjusted in RT compensation software.  
As many radiation thermometers do not compensate for the influence of atmospheric conditions, which is also the case for our Heitronics KT19.01 high temperature reference RT, the uncertainty of atmospheric conditions corresponds to change between calibration of the reference thermometer (at another institution) and subsequent use of this thermometer as the reference thermometer in calibrations at home institution. The laboratory calibration uncertainty (listed in column 5) is increased to account for the influence of the change in conditions.