On Rayleigh optical depth calculations

Many different techniques are used for the calculation of Rayleigh optical depth in the atmosphere. In some cases differences among these techniques c an be important, especially in the UV region of the spectrum and under clea n atmospheric conditions. The authors recommend that the calculation of Ray leigh optical depth be approached by going back to the first principles of Rayleigh scattering theory rather than the variety of curve-fitting techniq ues currently in use. A survey of the literature was conducted in order to determine the latest values of the physical constants necessary and to revi ew the methods available for the calculation of Rayleigh optical depth. The recommended approach requires the accurate calculation of the refractive i ndex of air based on the latest published measurements. Calculations estima ting Rayleigh optical depth should be done as accurately as possible becaus e the inaccuracies that arise can equal or even exceed other quantities bri ng estimated, such as aerosol optical depth, particularly in the UV region of the spectrum. All of the calculations are simple enough to be done easil y in a spreadsheet.