A novel device to measure atmospheric visibility

Abstract

An instrument which measures the amount of light scattered by cloud water droplets and subsequently calculates the extinction per metre (i.e. the atmospheric visibility) of the cloud is presented. The instrument has proven capable of measuring visual range in or out of cloud, is capable of measuring extinctions down to 0.0003 m-' and has operated continuously in the field for periods in excess of a year without servicing, providing remote continuous on-line monitoring of cloud conditions on a ten minute averaged basis. This forms an integral part of a larger project to calculate the annual sulphate and nitrate deposition at elevated sites.The primary objective of the instrument is to measure the extinction due to water droplets; however, it can be adapted to provide the approximate liquid water content appropriate to the extinction it measures. Apart from being a valuable research tool, it is also suitable as an early warning device in situations where detection of ground level cloud (i.e. fog) is important, e.g. on motorways, and may be used to activate other instruments which need to operate only under certain conditions.