When ice crystals and supercooled droplets are near each other, there is a movement of water molecules from the droplet to the crystal. This increases the size of the ice crystal at the expense of the droplet. This is because the saturation vapor pressure over ice is less than that over water. So if the air is saturated with respect to water, it is supersaturated with respect to ice. The supersaturation over the ice causes some of the water vapor to deposit on the ice crystal. This forces the liquid water out of equilibrium with the water vapor, and the liquid water evaporates giving the ice crystal a new supply of vapor to undergo deposition.