Background Information of Isohyet Chart

The isohyet chart for Hong Kong is obtained by integrating rain-gauge data and weather radar data.

More than a hundred of automatic rainfall stations exist in Hong Kong. They are operated by the Hong Kong Observatory, the Geotechnical Engineering Office and the Drainage Services Department. The rain-gauges automatically report rainfall measurements to the Observatory Headquarters through the network.

Data from the Observatory's weather radars at Tai Mo Shan and Tate's Cairn also provide estimates of rainfall amount.

The system producing the isohyet chart integrates the observed readings from the rain-gauges with the estimated rainfall from radar data (radar rainfall) and performs quality checks automatically, then the isohyet chart is generated by the computer analysis. In general over the areas that the rain-gauges are relatively dense, the rainfall amount is close to the observed readings; whereas over the region without rain-gauge, the computer rainfall analysis makes more reference to the radar rainfall. The method of computation can be referred to Reprint 950 or Reprint 1024.

Readers should beware that by its nature, the isohyet chart serves only to indicate visually the spatial pattern of rain. It should not be relied upon to derive exact rainfall amounts at specific locations. Furthermore, this is an automated product generated by computer for quick reference, erroneous data may slip through quality checks in spite of the best of our intentions.