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Thursday, 17th April 2014

Using 9-day weather forecast smartly

The Observatory rolled out its 9-day weather forecast service on 1 April this year. The period of forecast was extended by two days from the previous 7-day forecast. This enables the public to appreciate weather changes earlier so as to plan their activities ahead and get well prepared. You may notice that the webpage was given a facelift. It displays the forecast of temperature and humidity on colourful charts and incorporates related climatological information. Besides freshening up the look, these features aid the appreciation of weather changes and comparison with past climate.

You may find some red and blue shadings on the temperature and humidity charts. Can you guess what they are? They represent climatological information in the past. For example, the red shading on the chart represents the middle 50% range of the daily maximum temperature or daily maximum humidity within 30 years from 1981 to 2010 arranged in ascending order. Let us take 17 April as an example. We choose a five-day period from two days before to two days after 17 April, i.e. from 15 April to 19 April. We then select the same dates over the 30-year period, giving a total of 150 days. The maximum temperature or maximum relative humidity of these days were then sorted in ascending order and divided into 100 equal parts statistically. The value corresponding to the part ranked 25 is called 25th percentile, while that corresponding to the part ranked 75 is called 75th percentile. The red shading represents the range between these two percentiles, which accounts for 50% of the whole set of data. Therefore, if the forecast value falls inside the red shading, it means that the value is within the middle 50% range of the climatological data. If the forecast value is above the red shading, it is higher than the middle 50% range of the climatological data. Similarly, the forecast value is lower than the range if it is below the red shading.

By the same token, the blue shading on the charts represents the middle 50% range of climatological data of daily minimum temperature or daily minimum relative humidity.

Figure 1

Charts showing temperature forecast, humidity forecast and climatological information.


The accuracy of forecast generally decreases with forecast period. The formulation of weather forecast for the eighth and ninth days is mainly based on the output from numerical weather prediction models. The accuracy also varies for different weather systems in different seasons. Generally speaking, weather in spring and summer such as fog, tropical cyclone, thunderstorm and heavy rain is more changeable, resulting in higher uncertainty of forecast. Objective verification of various weather elements shows that the average accuracy of the weather forecast for the eighth and ninth days reaches about 80 per cent, which is comparable to that of the sixth and seventh-day forecasts newly launched back in 2004.

Figure 2

Accuracy of forecasts for the 8th and 9th days on par with that of the 6th and 7th-day
forecasts newly launched back in 2004.


When you check the 9-day weather forecast next time, other than the forecast values of temperature and humidity, please also take a look at their day-to-day variation and trend, and compare them with the climatological data. When the day of concern comes close, remember to check the latest weather forecast by the Observatory!



F.Y. Lee



Last revision date: <09 Jun 2014>