# 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 table in Appendix A captures the time required in each trial to obtain a steady-state wet-bulb temperature measurement for each instrument. In all trials, the Sling Psychrometer required at least two minutes of slinging at about two revolutions per second to arrive at a steady wet-bulb temperature. The Kestrel and IR Thermometer required much less time to measure a wet-bulb temperature--usually about 30 s--but the Kestrel did have a few trials that required more than 90 s to reach a steady wet-bulb temperature. In all trials, the Sling Psychrometer required more time to obtain a steady dry-bulb temperature than the Kestrel and IR method. Overall, using the IR Thermometer Psychrometer was the fastest way to measure wet-bulb and dry-bulb temperatures. The Kestrel did provide instantaneous moisture parameter (e.g., relative humidity, vapor pressure, dew point) calculations that were not automatically derived with the Sling Psychrometer or IR Thermometer Psychrometer. To obtain dew point temperatures from wet- and dry-bulb temperatures, psychometric calculations, such as those outlined in Section 2.2; psychometric tables; or a psychrometric whiz wheel, such as that shown in Figure 6, must be used. The Bacharach Sling Psychrometer has a psychrometric slide-rule as part of the instrument, but this simple tool assumes a constant (presumably sea-level) pressure and a constant \(L_{\circ}\), thus leading to inaccuracies. As such, the built-in Bacharach slide-rule tool was not used in this study. Appendix A lists the dew point temperatures as calculated from wet- and dry-bulb temperatures measured by the Sling Psychrometer and the IR Thermometer, or as read directly from the Kestrel. The dew point values are plotted in Figure 7 for all instruments with lines connecting the trial data points to allow the differences to be more easily seen. The Kestrel-displayed dew point temperatures were derived via the Kestrel's built-in algorithm. As is evident in Figure 7, the USAF whiz wheel and the psychometric equations in Section 2.2 provided virtually the same results. The USAF whiz wheel was predominantly used to obtain the dew point temperatures for the Sling Psychrometer. Figure 7 also shows the high wind scenario discrepancies noted previously. In particular, there were seven trials (32, 33-35, 37-39) in which high winds resulted in excessive evaporative cooling of the paper towel target and unreliable IR wet-bulb temperatures, which in turn led to poor dew point temperature and relative humidity calculations. The possible utility of using IR Thermometers/detectors to evaluate moisture and heat fluxes near the surface was also explored in this research. In separate experiments, it was found that the IR sensor could be used to sense wet- and dry-bulb temperature changes of 0.7 \({}^{\circ}\)C and 0.6 \({}^{\circ}\)C over vertical distances as small as 50 cm, respectively. The results of these experiments using the Kestrel, IR Thermometer and paper towel pieces are shown in Figure 8. Lines are used to connect the data points across the trial times only to allow the differences between the values of each instrument to be more easily viewed. As referenced earlier, Figure 5 shows the vertical placement of the paper towels, and all data are available in Appendix A. The plots in Figure 8 demonstrate that both the Kestrel and IR Thermometer are able to discern the expected daytime near-ground temperature gradient--in which, over transpiring grass, the dry-bulb temperature is expected to rise slightly with height (in the first meter [8]). However, the transpiring grass should also result  
Figure 7: Dew point temperatures measured by the Kestrel (black) and derived from dry- and wet-bulb measurements collected by the Sling Psychrometer (red), infrared thermometer (blue) and whiz wheel calculator (green). in greater moisture near the ground; therefore, the wet-bulb temperature should drop with height in the first meter above the surface [8].