Some electrical effects associated with the formation of precipitation elements

• Main Author: Matthews, John Brian
• Published: Imperial College London 1963
• Subjects: 551.57
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.602251

This thesis describes experimental investigations of two mechanisms of charge generation in the light of the requirements of a thunderstorm electrification theory. Attempts to measure the electrification produced by the melting of ice and snow known as the Dinger-Gunn effect have been made with three different experimental arrangements. In one of these, snow crystals were grown and melted under very clean conditions inside a diffusion cloud chamber. The results of the three experiments are consistent in failing to detect a separation of charge and in indicating that any charge produced was 4102 esu/gm or two orders of magnitude smaller than reported by Dinger and Gunn. This result was unaffected by varying the purity of the ice, its air content and the rates of freezing and melting and the pH value of the water. Accordingly, it is concluded that the charging associated with the melting of snow or hail is unlikely to be an important feature in the electrification of clouds and precipitation. An investigation of the distortion and break-up of freely falling water drops in electric fields in the range from 300 V/cm to 1500 V/cm is described. Large drops of water were produced and allowed to break up whilst freely falling in air. The drops were photographed during break-up 5 in an electric field by a high-speed tine camera and the electric charge carried by the fragments was measured. Factors influencing the break-up of water drops in steady and transient air streams are discussed and the critical conditions for rupture and deformation deduced. A theory for the expansion rate of a bubble formed by a freely falling water drop is discussed in the light of the experimental results and tentative suggestions made for the improvement of the theory. Charges separated by water drops breaking up in a field of about 300 V/cm are found to be about 1 esu/gm or two orders of magnitude larger than those used in Simpson's thunderstorm theory. The sign of the charge separated reverses as the field is reversed and the magnitude is proportional to the electric field. The mechanism is not very sensitive to impurity in the water. Accordingly, it is concluded that electrification due to the break-up of raindrops in an electric field may be important as a secondary mechanism in a thundercloud.