Although much evidence from laboratory and modeling studies along with field observations such as those from Explorer (Dye et al., 1986, 1988; Breed and Dye, 1989) had accumulated by the mid to late 1980s to suggest that the non-inductive ice-graupel collision mechanism was the dominant mechanism of electrification in cumulus congestus clouds and thunderstorms, the evidence was primarily circumstantial. There were few direct measurements of particle charge in association with particle size, type (ice or water) and shape to verify that charges on graupel or small hail agreed with what would be expected from the non-inductive mechanism nor directly refute the inductive mechanism. There had been only a few studies that had measured charge on precipitation particles and fewer yet that had simultaneous observations of size as well as charge. See Weinheimer et al., 1981, for a review of past studies.

Recognizing this need, the cloud physics group at NCAR developed for Explorer an instrument with the capability of simultaneously measuring charge on particles imaged by a Particle Measuring Systems 2-D imaging probe. A cylinder with two embedded induction rings were mated to a 2D-gray scale probe loaned to NCAR by Hugh Christian at NASA Marshall Space Flight Center. The induction ring cylinder was mounted directly behind the laser beam used to generate the images of particles. This effort followed similar developments by Vali et al., 1984; and Cupal et al. 1989. Weinheimer et al. 1991 describe the resulting NCAR instrument including a description of prior work on this topic. This comprehensive publication presents results from a noteworthy flight of Explorer over Langmuir Lab in New Mexico using this 2D-Q instrument.