mbs said:
The exact mechanism isn't well understood (likely there are many), but the creation of large quantities of ice particles is needed to create an electrical charge sufficient to generate lightning discharges inside a cumulonimbus cloud. There are also other mechanisms that can create lightning without ice particles present, volcanoes for instance. Actually volcanoes create multiple kinds of lightning. The steam and ash ejected directly out of the volcano vent can become charged at the get-go due to processes not understood. But volcanoes also produce "normal" lightning because a very large towering ash cloud also contains enough water and ice to act as a cumulonimbus cloud.
I saw a program on TV which showed scientists simulating what goes on in a thunderhead. They were investigating what sort of contact forces cause the electric field. I don't have any information other than what I remember. However, the little that I remember may be helpful.
Suppose that there is a collision between two equal size water droplets. The two particles coalesce into one particle because of surface tension, then the vibration shakes the single particle up so that it breaks up into two water droplets.
If the two final particles are equal size, then the two particles will be neutral. A symmetric collision results in a neutral charge.
If the two final particles are unequal in size, the two particles will have charges that are opposite in sign but equal in magnitude. Therefore, it is asymmetric collisions that separate electric charge.
The large droplets tend to sink due to gravity faster than the larger particles. Therefore, after a collision the two droplets are separated by gravity. This results in a potential difference between ground and air.
This was not on the program. However, think about snow flakes. The snow flakes don't have surface tension, so that they wouldn't coalesce as well as water droplets. Furthermore, ice is hard. A coalesced pair of snowflakes would have less of a chance of breaking up. Thus, snowflakes probably generate less electric potential than water droplets.
Maybe that is one reason lightning is more common in rain then in snow. There are fewer asymmetric collisions between snow flakes than between rain drops.