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A weather radar is mainly used to monitor the location,movement and-development of rain areas, and is particularlyuseful in locating the centres of tropical cyclones withinthe radar's range.The Royal Observatory began using radar for weatherobservation and forecasting in 1959. The first installationwas a Decca storm warning radar with a maximum range of250 nautical miles based on top of Tate's Cairn. Thisequipment was used for nearly 20 years, until June 1979.However, in December 1966, a second radar, with amaximum range of 240 nautical miles, had been installed atTate's Cairn.In early 1983 a new computer-based radar was installed atTate's Cairn. This equipment has three elements: the Tate'sCairn installation; a data processing station at the RoyalObservatory; and a remote display station at the airportmeteorological office.Computerised equipment at each station can produce aconstant altitude plane position indicator — basically adisplay of a horizontal cross-section of the atmosphere at aspecified altitude. Other special displays can also beproduced on a coloured screen. Different rainfall intensitiesare indicated by different colours.The radar antenna at Tate's Cairn is a 3.6 metre diameterdish. The sensitive receiver can detect rainfall rates aslow as 0.6 mm an hour at the radar's maximum range of450 kilometres.In normal operation the antenna scans helically upwardsat six revolutions a minute. At the maximum elevation of 37degrees, the antenna is lowered and the helical scan repeated.Microprocessors generate cross-sectional displays up to 20kilometres in height at one kilometre intervals. A microwavelink carries the data to the Observatory.At the Observatory, a minicomputer can generate one ormore of these special displays at the operator's command:• An echo top map showing the heights of the tops ofrain areas;123