Question

Clouds are white, but this is not because droplets are large compared to the wavelength, contrary to what many people think. Rather clouds are white for the same reason that milk is white. Cloud droplets scatter all wavelengths equally, while individual milk particles are small enough to scatter blue. What matters though for making both things white is that they both have a high optical depth. That is that multiple scattering leads to a washout of the color dependence. With reference to the equation show that if even if T - T(X), if r is small then the wavelength dependence of T (X) (i.e. dT/dX) can be seen in the reflectivity, but if T is large, then it can't. You can show this either by graphing, or (perhaps more easily) by using calculus to find dR/dX as a function of r for a small T and a large τ scenario

Answer 1