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

More than 90 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 rainfall stations automatically report rainfall measurements to the Observatory Headquarters through telephone lines.

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

For the production of the isohyet chart, estimated rainfall from radar data is correlated with the observed readings from the rain-gauges. In places where rain-gauges are not dense enough, rainfall amounts at specific points are estimated on the basis of the correlation.

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.