I have been involved in the issues of inflight icing for a number of years, both as a USALPA representative and an independent consultant. In the course of that work, I developed an icing database from a review of all US icing accidents and reportable incidents from 1978 through 2006. The latest iteration of this work was a presentation to the SAE at Seville in September of 2007, and can be seen at this URL:
http://www.sae.org/events/icing/pres...07s30green.pdf
Unfortunately, this online PDF has some problems with some of the charts; I'd be glad to send a cleaner copy to anyone interested.
One outcome of the work was based on normalizing the frequency of surface precipitation reports with the events in the database. 32% of the events were associated with snow, and snow comprised 32% of the surface precipitation reports in the continental United States. However, 33% of the events were associated with either ZR or ZL, which comprised only 1.8% of the total surface precipitation reports.
The majority of this data involves non-transport certificated aircraft.
It would be a mistake to assume that snow is safer, as seen by the large number of icing events that take place in the presence of snow. While the snow itself does not generate an ice accretion, it is often mixed with supercooled liquid water, which does. But the high frequency of events associated with freezing precipitation, compared to it's relative low occurrence rate, is an indication of it's vicious nature in the world of aerodynamics.
Indeed, there are not as yet any certification criteria or standards for conditions known as Supercooled Large Droplets (SLD) which we define as droplets with a median diameter of more than 50 microns. Thus, no aircraft are equipped with ice protection systems that have been designed or certificated for these conditions. This is in stark contrast to the existing holdover tables, which do allow operation in light ZR and light to moderate ZL. The dichotomy has kept a number of people in the authorities very busy, and very frustrated, for several years now.