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ABSTRACT
A simple analytical method was developed for directly calculating the thermodynamic wet-bulb temperature from air temperature and the vapor pressure (or relative humidity) at elevations up to 4500 m above MSL was developed. This methodology was based on the fact that the wet-bulb temperature can be closely approximated by a second-order polynomial in both the positive and negative ranges in ambient air temperature. The method in this study builds upon this understanding and provides results for the negative range of air temperatures (-17° to 0°C), so that the maximum observed error in this area is equal to or smaller than -0.17°C. For temperatures ≥0°C, wet-bulb temperature accuracy was ±0.65°C, and larger errors corresponded to very high temperatures (T^sub a^ ≥ 39°C) and/or very high or low relative humidities (5% < RH < 10% or RH > 98%). The mean absolute error and the root-mean-square error were 0.15° and 0.2°C, respectively.
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1. Introduction
First principles dictate that for any given ambient air mass, the difference between aspirated (well coupled) air temperature that includes ambient water vapor (drybulb temperature Ta) and the temperature of the same air mass (wet-bulb temperature Tw) at saturation provides a direct measurement of the amount of water vapor that air mass contains. This estimate can be determined as both relative and absolute quantities (Loescher et al. 2009). In other words, Tw is the temperature that a volume of air would have if cooled adiabatically to saturation at a constant pressure where all the heat energy came from the measured volume of air (Monteith 1965). The importance of this first principle is better realized when the Ta and Tw measured at both a surface level (boundary condition) and at different heights, that is, reference levels, can be used to estimate the evapotranspiration vertically through these two levels, for example, through a leaf or a canopy surface (Slatyer and McIlroy 1961; Alves et al. 2000; Balogun et al. 2002a,b).
Wet-bulb temperature is a basic hydrostatic, physical quantity that can be used to estimate basic physical weather parameters (Stull 2011). Some applied applications of Tw include linking surface and boundary layer flows (Wai and Smith 1998) and interpreting surface scalar fluxes using physical properties (Loescher et al. 2006),...