Raindrop charge and fall velocity measurements at a tropical station

Abstract

An instrument for measurement of raindrop charge and fall velocity has been designed, fabricated and field tested. The technique involves measurement of the static charges induced on two metallic cylinders as the raindrops pass through them. The metallic cylinders are installed one above the other along the same vertical axis and they were isolated with teflon insulators. The raindrop fall velocity was evaluated by measuring the time delay between the pulses obtained as the raindrops pass through the two cylinders. Observations of raindrop size and charge distribution were made at Pune during the rain showers from warm monsoon clouds. The raindrop size spectra follow the Marshall-Palmer distribution. The slope of the charge distribution is nearly equal to that of the size distribution of raindrops. A few positively charged raindrops of larger size arrive at the surface first followed by a large number of smaller size negatively charged drops. The magnitude of the positive charges was slightly less than that of the negative charges. The results and the electrical characteristics of warm clouds forming during the summer monsoon season are discussed.