The Observatory plans to give weather watchers extra information in its forecasts by attaching a probability rating for the predicted conditions. And it will soon launch the new feature on its online service for computers and mobile devices, beginning with rain. "There remains a question of whether the public will accept it," Observatory director Shun Chi-ming said yesterday. "But we hope we can explain to the public that forecasting errors can be very different in varying weather situations," he said. The Observatory has yet to decide how it will indicate the probability of rain - as a percentage, or with words like "high" or "low". Assistant director Cheng Cho-ming said Beijing once introduced a rainfall probability forecast but dropped it later as the public found it confusing. "It is open to interpretation what, say, a 40 per cent likelihood of rain really means," he said. Last year, the Observatory's digital platforms had 39 billion page views - five times that of 2011. Cheng said some sophisticated users might find the extra information useful to help them determine their risks in organising activities and to decide what to prepare for. Detailed rain probability information is given in weather reports in Japan and elsewhere. The Observatory will also extend its regional forecast from three days to seven, bringing it in line with the local long-range forecast. It predicts, meanwhile, that four to seven tropical cyclones will affect Hong Kong this year, within the normal range of five to six. Rainfall will be 1,900 to 2,500 millilitres, which is normal to slightly lower. Shun also warned of extreme weather such as heavy rain over a short period of time. Last year, the city experienced its first No10 typhoon signal in 13 years. Shun said the chance of another of such magnitude could not be ruled out but that it was beyond their ability to predict. Established in 1883, the Observatory will celebrate its 130th anniversary this year. A two-month exhibition, to be launched this weekend, will be held at the Museum of History. Shun said it was trying to borrow a six-inch Lee Equatorial telescope to exhibit. It had been used to report local time for ships through astronomical observations when the Observatory was founded.