Common sense suggests that the water in clouds will freeze when the temperature falls below zero degrees Celsius. However, our sense of such things is based on observations of masses of water where a single condensation nuclei can cause the entire mass of water to freeze. A cloud, on the other hand, is based on a large amount of small drops of water where each drop in a cloud requires its own condensation nuclei in order to freeze. However, large quantities of condensation nuclei are not likely to occur under even abnormal conditions and therefore clouds require temperatures far below the freezing point of water for significant amounts of ice crystals to develop.
Theory predicts that homogeneous deposition occurs only for extreme conditions of supersaturation.
The saturation conditions needed to satisfy this situation are more than 20 times the initial supersaturation state at a minimum. Therefore, homogeneous deposition at temperatures greater than 40 degrees C is rare given the necessary supersaturation conditions. The fact that liquid water droplets generally find a condensation nuclei long before strong saturation conditions occur is not only due to the presence of suitable ice nuclei but also on the fact that the saturation vapor pressure of ice is lower than that of water. That is, water droplets tend to deposit themselves onto the nearest, pre-existing ice crystals due to the difference in vapor pressures bypassing the need for a separate ice nuclei. It is noteworthy to mention the fact that developing clouds have a good correlation between the number of ice nuclei and the number of ice crystals in the cloud.