# Feasibility of Obtaining Surface Layer Moisture Flux Using an IR Thermometer
## 3 Discussion of Statistical Analysis and Measurement Results
### Dry- and Wet-Bulb Temperature Measurements over Grass, Concrete and Sand Surfaces  
The statistical data analysis of the experimental results shows that the IR Thermometer and wet paper towel technique described here can reliably quantify wet-bulb temperature compared to direct psychrometric and Kestrel wet-bulb measurements. Although cloud cover and a slight breeze provided ideal conditions, the IR Thermometers proved to be fast and effective for wet-bulb temperature measurements in nearly all conditions. Figures 2-4 show that dry- and wet-bulb temperatures measured by the IR Thermometer align well with the control Kestrel and Sling Psychrometer data sets, with an average difference of less than 5%. Note that data from the freezer experiments are not included in this <5% difference. Deviations from the control measurements can be partially attributed to solar heating contamination associated with man-made shading from direct sunlight, more specifically over the sand surface site due to the lack of shade and the utilization of a dark-colored (black) umbrella to shield the instrumentation from sunlight. Partly cloudy days also led to atmospheric and solar irradiance variability on time scales which likely were shorter than the time required to make each measurement. As stated previously, the total time to conduct all measurements over any of the particular surfaces studied was approximately 30 min, leading to plenty of opportunities for changes in sky conditions. Additionally, instances with high winds in the vicinity created notable fluctuations in IR Thermometer readings. For example, trials \(\sim\)30-40 were collected on 2 June 2020 when winds of 7.5 m s\({}^{-1}\) (15 kt) gusting to 10 + m s\({}^{-1}\) (20 + kt) were observed at a nearby airport and military base (see Appendix A for trial number to date/time mapping). The IR Thermometer dry-bulb temperature peaks seen in Figures 2a, 3a and 4a at trials 20, 47 and 69 (8 May, 3 June and 11 June) are directly attributable to high solar irradiance during those trials, affecting both the actual temperatures of the emitting surface and the internal temperatures of the IR Thermometer (despite attempts to maintain shade). Neither the Kestrel nor the Sling Psychrometer air (dry-bulb) temperature measurements were as affected at these times. However, the wet-bulb temperature plots in Figures 2b, 3b and 4b show much smaller differences among the Kestrel, Sling Psychrometer and IR Thermometer values at trials 20, 47 and 69. This suggests that the IR Thermometer dry-bulb temperature differences are due to the emitting dry paper towel surface becoming significantly warmer than the air temperature on sunny days, rather than the IR Thermometer losing accuracy due to solar-irradiance-induced temperature gradients within the IR Thermometer. As mentioned above, trials 30-40 (1 and 2 June) were affected by wind speeds higher than those usually recommended for stable Sling Psychrometer evaporative cooling. This is readily seen in the wet-bulb plots of Figures 2b and 3b (and relative humidity values as seen in Appendix A). The IR Thermometer values are significantly lower than those of the Kestrel and Sling Psychrometer. The authors postulate that the wet paper towel material can reach stable evaporative cooling at significantly lower ventilation rates than those required for a Sling Psychrometer because of the lower heat capacity and mass of the paper towel compared to the glass and alcohol in the Sling Psychrometer wet-bulb thermometer. Thus, although the high winds (up to 10 m s\({}^{-1}\)) that occurred during trials \(\sim\)30-40 were on the upper end of acceptable WMO ventilation rates for a Sling Psychrometer [2], such wind speeds were likely too high for the wet paper towel to remain at a steady temperature if water continued to evaporate from the material. It appears that ventilation rates of less than 5 m s\({}^{-1}\) are more favorable for the IR Thermometer Psychometric method described in this work.