Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83 SMI; site husky.uucp Path: utzoo!linus!philabs!cmcl2!harvard!seismo!rochester!ritcv!husky!mls From: mls@husky.uucp (Mark Stevans) Newsgroups: net.sci Subject: Re: Question about Electricity Message-ID: <262@husky.uucp> Date: Tue, 5-Nov-85 12:24:41 EST Article-I.D.: husky.262 Posted: Tue Nov 5 12:24:41 1985 Date-Received: Fri, 8-Nov-85 06:16:29 EST References: <621@hlwpc.UUCP> <662@petrus.UUCP> <714@alberta.UUCP> <554@ttrdc.UUCP> Organization: Eastman Kodak Company, Rochester, NY Lines: 44 The question of how atmospheric electricity is generated has been raised by the referenced article: > Now the question > of how the cloud achieves that voltage is a different one and I understand > there are several explanations (anyone care to elaborate?). Here is the best theoretical explanation of cloud electrification that I know of: Clouds are made up of droplets of water. If there is a lot of water in the cloud, larger droplets may fall out of the cloud as rain. If the cloud is relatively calm, the droplets usually stay in one piece as they travel about. In stormier conditions, water droplets break up and recombine as they get blown about in the cloud. There are some free electrons in every droplet of water. They naturally collect around the outside of the droplet, due to mutual repulsion. If a droplet begins to break up into two sub-droplets, the electrons tend to stay on the larger droplet (statistically and fluid-dynamically speaking, droplets usually don't break up into exactly equal sub-droplets), because during this period (envision the process as similar to a yeast cell budding) the larger sub-droplet has less surface area per unit volume. We end up with a relatively positively charged small sub-droplet, and a negatively charged large sub-droplet. Due to their greater weight to surface area ratio, the large, negatively charged droplets tend to sink in the cloud, perhaps hitting the ground as rain, while the small droplets stay suspended in the air currents. Since the free electrons in the cloud are moving to lower altitudes, a potential difference is generated. Most lightning strokes are between upper and lower parts of a single cloud, and not between cloud and ground. The potential difference per unit distance increases until it is sufficient to ionize a "finger" of air about an inch wide. This highly conductive finger, which typically snakes from a positively charged region towards a negatively charged region, lengthens at a speed on the order of a thousand miles per hour. When the end of the finger gets close enough to its target, the potential difference between the end of the finger and the target causes free electrons on the target to arc across the gap into the ion finger. This "return stroke" is what we know as lightning. Mark Stevans ritcv!husky!mls