A Study of Ångström’s turbidity parameters from solar radiation measurements in India

Values of Ångström’s turbidity coefficient β and the wavelength exponent ∝ have been computed for a number of stations in India from pyrheliometric measurements of direct solar radiation, for the whole spectrum and for specified spectral regions using filters OG₁, RG₂ and RG₈. Large seasonal variations are noticed in β, with high values in summer and low values in winter at all stations. Rainout and washout are effective in the removal of aerosols from the atmosphere and a marked fall is noticed in β after thunderstorms and after the onset of the monsoon at all stations.

Values of ∝ at stations in the southern half of the subcontinent remain more or less constant throughout the year with a mean value of about 1.0, indicating that smaller haze particles predominate and the size distribution remains the same despite the large increase of turbidity in summer. In the northern half of the sub-continent, ∝ shows seasonal variations with low values, sometimes becoming zero or negative in summer and the normally accepted values in winter. Over north and central India therefore, while smaller particles are more numerous in winter, large particles predominate in summer.