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  • Knowing more about Very Hot Weather Warning

  • Wednesday, 24th August 2016

After more than a week of rainy days, the weather in Hong Kong finally turned fine but became rather hot this week. We all know that the Observatory will issue warning in case of very hot weather. However, you may recall there was occasion when the warning was in force, the maximum temperature recorded at the Observatory throughout the day did not reach 33 degrees. Why is this the case?

Figure 1

The Observatory will issue warning when the weather is very hot


As a matter of fact, the issuance of Very Hot Weather Warning is not only based on the temperature recorded at the Observatory headquarters in Kowloon, but also those recorded in the weather stations on the Hong Kong Island, the New Territories and outlying island, as well as the Hong Kong Heat Index which represents the combined effect of temperature, humidity, wind speed and solar radiation. Generally speaking, when the index reaches around 30, it means that the heat may cause adverse health effects. Taking 3 June this year as an example, the maximum temperature recorded at the Observatory was 32.4 degrees, but the Hong Kong Heat Index reached 30.9 (see diagram below) owing to abundant sunshine, moderate winds and slightly humid weather. The Very Hot Weather Warning was in force that day and the actual weather did meet the criterion of warning.

Figure 2

Hong Kong Heat Index recorded on 3 June 2016


Checking the records since 2014, we found that among all days with the Very Hot Weather Warning in effect, the actual weather met the warning criterion on around 80 percent of the days. Such performance is considered good as far as weather forecast is concerned. It is worth noting that the Observatory also provides Hot Weather Special Advisory, the issuance of which is based on basically the same factors as the Very Hot Weather Warning but at a different degree. It is usually issued when the weather conditions do not warrant the issuance of Very Hot Weather Warning but the hot weather might cause adverse effects to the health of the public.

When you engage in outdoor activities in summer, please take note of the Very Hot Weather Warning or Hot Weather Special Advisory and take precautions to avoid heat stroke or sunburn.



L.S. Lee    Y.H. Chan



Last revision date: <21 Sep 2016>