CHAPTER 4 HEAT BALANCE OF BARE GROUND

62 CHAPTER 4RD = (A + B * sqrt(WO)) * SIG * ATA 4 (4.16)where RD is downward long wave radiation (atmospheric radiation) (W/m 2 ), A andB are empirical constants related to each location with values of 0.53 and 0.06,respectively, for various locations in U.S.A., WO is humidity ratio (kg/kg DA), andATA is absolute air temperature (K) at the screen height. Since A and B aredependent on temperature and water vapor of the atmosphere, these constants mustbe determined at the place where this equation is applied.Figure 4.13. Long wave radiation from the sky (after Gates, 1962).A similar empirical equation was reported based on observation at six locationsin the U.S.A. by Martin and Berdahi (1984):RD = EPSA * SIG * ATA 4 (4.17)EPSA = 0.711 + 0.56 * TD / 100 + 0.73 * (TD / 100) 2 (4.18)where EPSA is atmospheric emissivity and TD is dew-point temperature ( o C) at thescreen height. Emissivities of various materials are summarized in Table 4.1.