During the summer of 1976 the ONR/NMIMT research aeroplane was employed in studies of the electrical properties of thunderstorms. Flights through clouds were made in Florida, as part of the Thunderstorm Research International Project, and in New Mexico. The most important measurements were of electric field and the charge, Q, and size, d, of individual precipitation elements. A novel device was constructed for the Q and d measurements. The charge carried on a particle passing through a metal cylinder was sensed by induction, and its size by a shadowgraph technique involving a linear array of photo-diodes. The penetrations were generally through the lower regions of the clouds.
The major findings of the studies in New Mexico were as follows:
1
Volume charge densities on precipitation, pp, were often around - 5nCm −3 over horizontal distances of several kilometres. pP was almost always negative, but positive charge densities, of lower magnitude, were occasionally observed over shorter distances. The major contribution to the measured values of pP was made by particles of size around 1 mm, or smaller.
2
Simultaneous measurements of Q and d showed that no simple relationship existed between them. Charges of about 100pC were commonly observed on particles around 1 mm in size. These are much too high to be explicable in terms of the inductive theory.
3
Positive and negative charges were found to coexist, except when the precipitation rate, p, was very low. However, charge of one sign (almost invariably negative) was always strongly dominant.
4
Values of p could be estimated crudely from the d pulses. In regions of high pP they were rarely in excess of 10mmh−1; on some occasios when pP was substantial p was below 1 mmh−1.
Our primary conclusions are that in the clouds studied substantial currents were often carried on precipitation, and that the charges on individual precipitation elements are not explicable in terms of the inductive mechanism of thunderstorm electrification.
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