Physical and Optical Properties of Fog at 74 Wavelengths from 0.35 to 90μm within a Sample of 239 Spectra

Abstract

Mean radius (r) and moments around r, total concentration of droplets, liquid water content (W), terminal fall speed and visual range (Vm), have been computed for a sample of 239 droplet size distributions of valley, advection and radiation fog. The scattering, absorption and extinction coefficients, the albedo for single scattering, the phase function P(cos *) at several scattering angles 0, the coefficients xn, x*m connected with the Legendre series expansion and the *-M approximation of the phase function have also been computed at seventy-four wavelengths from 0.35 to 90*m.Data elaboration has basically consisted in the determination of the standard statistical parameters (histogram, mean, value, dispersion, skewness and kurtosis) for each of the above quantities and each of the various fog groups considered.Results concerning the grouping of fog spectra according to the fog type show that all the quantities are widely dispersed within each fog group and, in addition, the respective histograms partially overlap to each other; similar results also emerge by grouping the spectra according to their values of W and V m, so that it appears unreliable to consider mean values as characteristic of a given fog group. Finally, a description of the mean physical and optical properties of the whole sample is presented, with the spectra all grouped together.

Phase Function Mean Properties at Visible and Infrared Wavelengths within a Sample of 239 Fog Spectra

“…In a previous study published in this Journal (Tonna, 1989a) we computed mean optical quantities within a sample of 239 fog spectra in the * =0 .35-90 *m wavelength interval, by grouping at first the spectra according to the fog type (valley, advection and radiation), then according to their liquid water content W, and finally to their connected visibility Vm; the range of W and Vm was divided in three intervals. In all three instances results showed that the computed quantities were widely dispersed within each fog group and that, in spite of different mean values, the relevant frequency distributions partially overlapped to each other, so that it appeared unreliable to consider mean values and behaviors of the computed quantities for each …”