psih Function

                                                                        *
 Calculation of the stability correction term                           *
                                                                        *
 AUTHOR: Matthias Langer, adapted by Andreas Stohl (6 August 1993)      *
         Update: G. Wotawa, 11 October 1994                             *
                                                                        *
 Literature:                                                            *
 [1] C.A.Paulson (1970), A Mathematical Representation of Wind Speed    *
       and Temperature Profiles in the Unstable Atmospheric Surface     *
       Layer. J.Appl.Met.,Vol.9.(1970), pp.857-861.                     *
                                                                        *
 [2] A.C.M. Beljaars, A.A.M. Holtslag (1991), Flux Parameterization over*
       Land Surfaces for Atmospheric Models. J.Appl.Met. Vol. 30,pp 327-*
       341                                                              *
                                                                        *
 Variables:                                                             *
 L     = Monin-Obukhov-length [m]                                       *
 z     = height [m]                                                     *
 zeta  = auxiliary variable                                             *
                                                                        *
 Constants:                                                             *
 eps   = 1.2E-38, SUN-underflow: to avoid division by zero errors       *
                                                                        *