Use Equation (4.12) to show that, at low microwave frequencies, the absorption coefficient of a cloud of water droplets is given approximately by

(i.e. independent of the droplet size), where ris the mass of water in kg per m³ of cloud. Assume that, for water at microwave frequencies, ε_p = 75.9, εˆž = 4.5 and

'. )dBkm GHz, 0.5 -3 kgm'