The coefficient C in Trabert's visibility equation, V = (Cρr̄)/W, where r̄ is linear mean radius, ρ density, and W is liquid-water content, is shown to be a function of the breadth of the drop-size distribution but has preferred values in natural clouds ranging from 3.3 for fair-weather cumulus to 4.8 for nimbostratus. The frequently quoted value of 2.6 pertains only to a perfectly monodisperse distribution, and is inappropriate in nature. A more useful equation is found to be V = (Kρd0)/W, in which >d0 is the median volume diameter and the coefficient K is found to be very nearly independent of the breadth of the drop-size spectrum over the range which occurs in natural clouds. K = 1.2 is shown to be an excellent value for 65 observations by Diem. Similarly, the coefficient G in the radar-reflectivity equation, Z = (6/π)G d03(W/ρ) × 10−6 has a preferred value of 1.35. The implication is that nature has a preference for a particular type of drop-size spectrum.