Is friction the cause of static buildup which leads to lightning?
A primary factor in lightning generation is an updraft, which can be produced in a number of ways. An updraft may be caused by a sea breeze, mountains, cold front, or other methods that produce instability in the vertical. The updraft must reach levels colder than freezing at its top. In the U.S. in summer, that’s about 15,000 to 20,000 feet above sea level. When the updraft carries liquid water to that height, some droplets turn to ice. The hail or soft hail, called graupel, becomes heavier than the water droplets, and starts to fall faster than the water, which may still be going upward. So the hail is carrying one amount of charge down through the raindrops that have another charge. This is the primary factor for lightning formation. The exact way this happens is still not completely understood. Basically, the more water droplets that are carried quickly up to levels with sub-freezing temperatures, the more lightning is produced.
