Several effects of turbulence may reduce the time for warm rain initiation. This talk focuses on the effect of local clustering  of inertial droplets that interact hydrodynamically. Using the  so-called "drift-diffusion" model (based on the Master equation satisfied by the probability of finding two droplets separated by a  given distance) it is shown that hydrodynamic interactions have strong effects on clustering of inertial droplets at sub-Kolmogorov length scales. This is an interesting problem for future investigations.