In its early days, as a system devised for the service of mariners, it relied upon wind speeds measured around Victoria Harbour. As the emphasis shifted to serving the intensively urbanised populace, such observations were seen as lacking relevance. The Hong Kong Observatory was the target of criticism during Typhoon Prapiroon in 2006 when conditions in urban areas were much more severe than those on the harbour which had justified only a No. 3 signal being issued. In response, from 2007, the Hong Kong Observatory broadened its network to eight near sea-level reference anemometers around Hong Kong. According to the new system, the No. 3 and No. 8 signal will be issued when half or more anemometers in the reference network register sustained winds of 41-62km/h and 63-117km/h respectively. In 2013, Lau Fau Shan has replaced Wetland Park as a reference anemometer station. The current eight reference anemometer stations include Cheung Chau, Hong Kong International Airport, Sai Kung, Kai Tak, Lau Fau Shan, Tsing Yi Shell Oil Depot, Sha Tin and Ta Kwu Ling.