By recording the time variations similar to those in Fig. for several different sizes, the relation between the appearance time and droplet diameter was obtained as shown in Fig. . The slope of the linear relation shown in Fig. gives the droplet growth rate in D, where is the droplet diameter. From this rate, the water condensation flux, ϕ (mol m -2 s -1 ) is derived in the following way. The increase in the droplet volume, ∆V, between time t 1 and t 2 is related to ϕ as