Abstract:
Anomalous properties of the surface of ice crystals have been extensively recorded in the last century. Latterly the presence of ice crystals in thunderclouds has been examined to determine whether sufficient charge is separated during collisions to account for lightning strokes. A qualitative theory is derived which predicts the dependence of charge separation during collisions upon certain environmental parameters, specifically air pressure, if the separation is connected with the existence of the other anomalous properties of ice surfaces. Experimental results show that the charge separated on to an ice target is independent of the charge on an incoming piece of ice and is independent of temperature. This result is at variance with a number of established cloud-electrification models.
For the controlled contacts of an earthed ice pellet repeatedly hitting an ice target, the ratio of standard deviation to mean of charge on the target is smaller than for the reported cases of free-falling ice hitting a large target. This is consistent with the model in which it is shown that the particular local properties of contact sites govern the charge separation. That is, repeated contacts at the same point would be expected to produce more regular results than for contacts at variable sites.
For air pressure variation from atmospheric pressure to about 20 mbars from -5 to -40°C, no consistent variation of charge separation with pressure was seen except for two occasions at -5°C. The mean charge decreased by about 30 percent for pressures from 400 to 20 mbars. This is only qualitatively consistent with the prediction from the model of charge separation presented earlier.