Ice crystals
Last night I tuned into the last part of a TV documentary filmed in Australia dealing with thunder storms and the size of ice crystals in ice clouds near and around these systems, but particularly in the cap of a cell. While no definitive conclusion was reached, it was newly discovered that crystals can vary in size by considerable amounts, with corresponding different effects on pitots tubes. Two of their test airplanes while attempting to collect samples, experienced loss of airspeed indications, and had to make emergency landings in the middle of these storms. I am not sure when these tests were done, as I say, I tuned in at the end of the film. Perhaps someone else can provide additional info