# Feasibility of Obtaining Surface Layer Moisture Flux Using an IR Thermometer
## 2 Methodology
### Conversion to Relative Humidity and Dev Point Temperature  
Relative humidity and dew point temperature can be calculated using psychrometric relationships based on data obtained from the IR Thermometer and Kestrel measurements. Assuming the same temperature and pressure, relative humidity is a ratio of the vapor pressure of the air to the saturation vapor pressure [11],  
\[RH=\frac{e}{e_{s}}\, \tag{2}\]  
[PERSON]'s equation [12] is used to determine the saturation vapor pressure (hPa):  
\[e_{s}(T)=6.112\exp\left(\frac{17.67T}{T+243.5}\right), \tag{3}\]  
with \(T\) being dry-bulb temperature (\({}^{\circ}\)C) measured by the IR Thermometer. Vapor pressure is calculated using the psychrometric equation [13],  
\[e=e_{T_{w}}+\frac{c_{pd}P}{L_{v}c}(T_{w}-T)\, \tag{4}\]  
where \(c_{pd}\) is specific heat of dry air (J kg\({}^{-1}\) K\({}^{-1}\)) at constant pressure, \(P\) is the barometric pressure (hPa) measured by the Kestrel, \(L_{v}\) is the latent heat of vaporization (J kg\({}^{-1}\)), \(\varepsilon\) is the molar mass of water over the molar mass of air (0.622), \(e\) is vapor pressure (hPa), \(e_{T_{w}}\) is the wet-bulb vapor pressure (hPa), \(T_{w}\) is the wet-bulb temperature (\({}^{\circ}\)C) and \(T\) is the ambient air or dry-bulb temperature (\({}^{\circ}\)C). Latent heat of vaporization tables were used to determine \(L_{v}\) values based on IR Thermometer measurements.  
Figure 3: (**a**) Individual observations, as well as the daily means and standard deviations (superimposed on the final daily observation) of DB temperatures measured by the Kestrel, Sling Psychrometer and IR Thermometer over grass. (**b**) Individual observations, daily means and standard deviations of WB temperatures measured by the Kestrel, Sling Psychrometer and IR Thermometer over grass.  
Figure 4: (**a**) Individual observations, as well as the daily means and standard deviations (superimposed on the final daily observation) of DB temperatures as measured by the Kestrel, Sling Psychrometer and IR Thermometer over sand. (**b**) Individual observations, daily means and standard deviations of wet-bulb (WB) temperature as measured by the Kestrel, Sling Psychrometer and IR Thermometer over sand.  
Figure 5: Moisture flux experimental setup consisting of small paper towels positioned at various heights (28, 46, 64, 81 and 99 cm) above a grassy surface.  
Dew point temperature (\(T_{d}\) in \({}^{\circ}\)C), the temperature at which vapor pressure (or mixing ratio) and saturation vapor pressure (or saturation mixing ratio) are equivalent, is calculated using the vapor pressure from Equation (4) and the following equation [12]:  
\[T_{d}=\frac{243.5\ln\left(\frac{e}{6.112}\right)}{17.67-\ln\left(\frac{e}{6.112} \right)}\, \tag{5}\]  
In addition to the measurements and equations outlined above, a USAF psychometric whiz wheel calculator (ML-322/UM), shown in Figure 6, was used to validate the use of an empirical Clausius-Clapeyron Equation (5) to derive dew point temperature from the IR Thermometer's dry and wet-bulb temperature measurements. Relative humidity and vapor pressure are also outputs from the whiz wheel tool, which is an accurate representation of the full psychometric equations on a series of circular slide rules based on the following inputs: dry- and wet-bulb temperatures, as well as ambient atmospheric pressure. One limitation of the whiz wheel tool occurs with very large wet-bulb depressions (the T\({}_{\text{drybub}}\)--T\({}_{\text{wetbub}}\) difference); this is due to the logarithmic nature of the scales and difficulty setting measurements on those scales.