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Following is a written reply by the Acting Secretary for Commerce and Economic Development, Mr Gregory So, to a question by the Hon Frederick Fung in the Legislative Council today (March 3):
Question:
Some members of the public have relayed to me that the forecast time of the southward migration of a cold front, the seven-day weather forecasts and weather forecasts for even shorter periods made by the Hong Kong Observatory (HKO) during the Chinese New Year holiday this year (from February 11 to 20) significantly deviated from the actual situations, and that the temperature forecasts had been amended time and again, while the actual temperatures on several days were significantly lower than those forecast. In this connection, will the Government inform this Council:
(a) during the aforesaid period, how the forecasts for the arrival time of cold front, and the maximum and minimum temperatures (including seven-day weather forecasts and local weather forecasts) compare with the actual situations, and of the time when cold weather warnings were issued;
(b) of the absolute accuracy scores (out of a maximum of 100) for the daily forecasts made by HKO during the aforesaid period, and of the reasons for the significant deviations of the aforesaid forecasts and actual situations in respect of the cold front; and
(c) of the method currently adopted by HKO for forecasting the arrival time of cold fronts and subsequent temperature falls; whether the authorities will review the current method of forecasting in view of the aforesaid deviations of the forecasts from actual situations, and assess afresh the impact of such large-scale weather systems (for example, winter monsoon) on local temperatures, as well as making corresponding improvements in hardware and software, so as to avoid the occurrence of similar situations as far as practicable?
Reply:
President,
The seven-day weather forecast of the Hong Kong Observatory (HKO) seeks to provide the public with an overview of the weather conditions for the coming week, in terms of general trend. HKO will update the projections in their shorter term forecasts in the light of the latest weather conditions.
Due to the inherent nature of changing weather conditions and the limitations of prevailing technology, the phenomenon that the projected temperatures in the seven-day forecast differ from the actual measurements is almost inevitable. Such a phenomenon is not unique to Hong Kong. In the winter months, Hong Kong's temperature is affected by factors including the strength of the cold air heading south, cloud cover, humidity and rainfall. As these factors are erratic in nature, it is not easy to accurately predict their interactions several days ahead.
With reference to the questions raised by the Hon Fung, our detailed reply is as follows.
(a) On February 5, 2010, HKO projected that a cold front would pass through Hong Kong during the Lunar New Year holidays. Accordingly, HKO mentioned in its forecast on that day that there would be a significant temperature drop on February 12. As forecast, HKO issued the Cold Weather Warning at 4.20pm on February 12. The warning was cancelled at 4.20pm on February 13.
On February 11, HKO forecast that the northerlies would strengthen and announced in the forecast on that day that the temperature would drop further within a few days. HKO issued, as forecast, the Cold Weather Warning again at 4.20pm on February 14. It was cancelled at noon on February 20.
Tables comparing the minimum and maximum temperature forecasts (ranging from seven-day to one-day forecast) with the actual measurements recorded at HKO's Headquarters during the period between February 11 and 20 are at Annexes A and B respectively. As shown in Annex A, the minimum temperature shown in the Local Weather Forecast issued one day before was the same as the actual measurement on some days. On some other days, it was higher or lower than the actual measurement but the margin did not exceed one degree on any occasion.
(b) HKO assesses the accuracy of its weather forecasts by comparing the differences between the forecasts and the actual outturn in terms of a basket of factors, including temperature, wind speed, cloud cover, visibility and precipitation. HKO does not produce accuracy scores for temperature forecasts alone.
(c) In making seven-day weather forecasts, HKO needs to take into account the weather conditions in neighbouring areas far beyond Hong Kong. HKO mainly relies on the mathematical output of the global weather change projections generated by the computers of overseas meteorological centres (including Europe and Japan). HKO also makes reference to the data provided by nearby meteorological authorities.
Since the shorter term forecasts are of greater relevance to the daily lives of the general public, HKO attaches more importance to such forecasts. In making these forecasts, HKO draws reference from the above data and would also make use of the readings from its own equipment and the mathematical algorithms developed by departmental colleagues on the basis of the experience acquired over the years.
HKO will continue to keep abreast of advances in technology relating to temperature forecasts. Where appropriate, HKO would consider upgrading its hardware/software for the purpose of enhancing its forecasting service.
Ends/Wednesday, March 3, 2010
Issued at HKT 17:06
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