The free electrical charge on thunderstorm rain and its relation to droplet size

Using a new induction type of apparatus, simultaneous measurements were made of the free electrical charge and the mass of raindrops falling from active thunderclouds. The free charge Q on raindrops of radius a is given by Q/a2 = ES, where ES is the electric field at the surface of the droplet. Eighty per cent of the measured droplets had values of ES lying between ES = 1 esu/cm and ES = 10 esu/cm, with an average value of ES = 2.85 esu/cm. The average measured free charge was 0.015 esu per drop for the positives and 0.019 esu per drop for the negatives. Negative droplets were more massive than the positives. After two droplets of one sign fell, there was a high probability that the next droplet would be of opposite sign.