A NOVEL DEVICE TO MEASURE ATMOSPHERIC VISIBILITY

An instrument which measures the amount of light scattered by cloud wa ter droplets and subsequently calculates the extinction per metre (i.e . the atmospheric visibility) of the cloud is presented. The instrumen t has proven capable of measuring visual range in or out of cloud, is capable of measuring extinctions down to 0.0003 m(-1) and has operated continuously in the field for periods in excess of a year without ser vicing, providing remote continuous on-line monitoring of cloud condit ions 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 meas ure the extinction due to water droplets; however, it can be adapted t o provide the approximate liquid water content appropriate to the exti nction it measures. Apart from being a valuable research tool, it is a lso 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 u nder certain conditions.