1. The problem statement, all variables and given/known data Hi everybody. I am currently trinyg to solve the first exercice of the fifth chapter of R.R. Rogers book about cloud physics. "Show that the vapor pressure in equilibrium over a pure water drop of radius r decreases with T if r<2σ/LρL. 2. Relevant equations es(r)=es(r=infinite)*(2σ/rRvρLT). This equation relates the vapor pressure of saturation of a drop of radius r and the one from a plain surface. es is the vapor pressure, r is radius, σ is surface tension, Rv is the gas constant, ρL is density and T is temperature. 3. The attempt at a solution As far as I understand, I have to derivate es(r) respect to T and see if the result is similar to the crtical radius which value is 2σ/RvρLTlnS. If I have understood correctly, none of the values in the expression of es depends on T, therefore d/dT=∂/∂T. If I derivate exp(a/T) I obtain -a*exp(a/T)/T2. This expression doesn't look like the one expressed in the upper paragraph. Note: a=2σ/rRvρL If someone could give me some guidance I would be extremley grateful.