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  • Why do we feel much colder on a windy and rainy winter day?

  • Friday, 23rd January 2015

In winter, Hong Kong is frequented by the cold monsoon. You may notice that one feels colder on a windy and rainy day than a day at the same temperature but in fine weather or light wind. Why is there such a difference?

Our body produces heat continuously by metabolism in order to maintain a body temperature of around 37 degrees Celsius. When the ambient temperature is lower than the body temperature, body heat is lost to the environment, making one feel cold. Apart from air temperature, other meteorological conditions such as wind speed, humidity and solar radiation also affect the overall rate of heat loss. Since air movement helps carry away our body heat, we feel particularly cold under windy conditions when heat loss is faster. This phenomenon is called the 'wind chill effect'. Besides, we also feel particularly cold on rainy days. This is because water has higher heat conductivity than air and hence water droplets on our body can take away the body heat more efficiently. On the other hand, when we are exposed to sunshine on a fine day, the heat absorbed from solar radiation offsets part of the heat loss from the body, thus making us feel relatively warmer. On a gloomy day, however, we feel just the opposite. Therefore, if we are facing the winter wind and rain, we will have a stronger feeling of coldness. The temperature we perceive, commonly known as the apparent temperature, will be lower than the actual air temperature measured by instruments.

Depending on their climates, some countries devise wind chill index or apparent temperature, which specifically considers the heat transfer processes involved between the human body and the surrounding environment, as well as the responses of the human body tested under different meteorological conditions in a cold environment. However, there is so far no international standard on the calculation method and the applicable range of such wind chill indices. For example, the index currently used in the U.S.[1] is only applicable when the actual air temperature is 10 degrees Celsius or below, while in Canada[2] the index is to be used only at or below 5 degrees Celsius. It is noteworthy that the aforesaid indices normally do not include the effects of sunshine. As such, when there is sunshine in winter, such indices will be lower than the actual perceived temperature.

The climate in Hong Kong is different from the countries which widely adopt the wind chill indices. This is because there are not many days when air temperatures generally fall below 5 or 10 degrees Celsius. Hence the indices used in other countries may not be entirely applicable to Hong Kong. As compared to countries where snowstorms are common in winter, the wind chill effect in Hong Kong, especially in urban areas, is not particularly significant. For most of the time the perceived temperature does not differ much from the air temperature measured by instruments. If the apparent temperature is reported along with the actual air temperature, the public may get confused with the two similar numbers. To remind the public of the cold conditions in a simple and clear way, the Observatory now issues the Cold Weather Warning where the situation warrants, that has taken into account the combined effects of air temperature, wind speed and humidity, and reminds the public to take appropriate measures to keep themselves warm.

HO Chun-kit


[1] "NWS Winter Storm Safety: Windchill Information and Chart" (, NOAA National Weather Service

[2] "Canada's Wind Chill Index" (, Environment Canada

  • The Tolling of Climate Alarm Bells

  • Wednesday, 17th December 2014

"No man is an island, entire of itself;
every man is a piece of the continent, a part of the main......
any man's death diminishes me, because I am involved in mankind,
and therefore never send to know for whom the bell tolls;
it tolls for thee."  -  John Donne (1572 - 1631)

Twenty-four years ago when the Intergovernmental Panel on Climate Change (IPCC) published its First Assessment Report, it was not certain then whether the observed global warming was due to natural climatic variations or human influence. With the advent of climate science and the increasing amount of observations and evidences, the sure sign of human influence became increasingly too obvious to ignore and deny. By the time of the Third Assessment Report in 2001, it was concluded that warming due to natural causes was very unlikely. Another decade or so had gone by and then came the strongest message ever spoken by science as the final instalment of the Fifth Assessment Report, the Synthesis Report (Figure 1), was released in November 2014 [1]. The proclamation of human influence on the Earth's climate was loud and clear!

Figure 1

Figure 1      The Synthesis Report of IPCC's Fifth Assessment Report

As the certainty in observing, detecting and attributing the warming climate increased over the past couple of decades, IPCC also accentuated its language in warning the possibility of a grim reality within this century marked with more frequent extreme weather events and significant sea level rise if greenhouse gas emissions continued unabated. However, the world was slow to respond to such warnings as the atmospheric concentration of carbon dioxide continued to climb and hit its highest level in the past 800,000 years (see the blog "Record High Carbon Dioxide Concentration"). For a coastal city like Hong Kong which is vulnerable to an increased risk of sea flooding as a result of sea level rise and enhanced storm surges brought by tropical cyclones, planning and infrastructure designs for sustainable development need to ensure sufficient resilience against such potential impacts.

Nature has sounded the alarm bells and is doing its utmost best to wake us up. Globally, 2014 is very likely to be the warmest year on record [2]. Hong Kong has also just experienced record-breaking summer months this year with the average temperature of June-September reaching the highest since records began in 1884. Simple law of physics tells us that warm air has the capacity to hold more moisture. So when it rains, it pours! A heated land surface under high temperatures will also trigger more heavy rain and thunderstorms. We are already seeing both trends here in Hong Kong. Hourly rainfall records used to take several decades to be broken; but new records have been set several times in the past couple of decades, and the latest by a wide margin (Figure 2). And it is also official that thunderstorm activities in Hong Kong have been the most frequent ever in 2014, with 59 thunderstorm days registered up to the end of November, already exceeding the thunderstorm days record for the whole year.

Figure 2

Figure 2      Hourly rainfall records at the Hong Kong Observatory headquarters in Tsim Sha Tsui (1885 - 2013)

Shortly after the release of the Synthesis Report came the joint China-U.S. announcement of reduction targets of greenhouse gas emissions [3]. Despite the continual difficulties in the negotiation among governments as recently revealed at the Lima Climate Change Conference, hopefully the joint China-U.S. announcement could bring more positive developments to the next round of governmental negotiations in Paris in 2015. But while we all hold our breath for the governments of this world to come to terms with the inconvenient truth, we as citizens of Planet Earth also have the option of taking matters into our own hands by adopting a low-carbon or zero-carbon lifestyle, which requires a fundamental change in our excessive consumption behavior and wastage. And we need to change now, as climate change is already a clear and present danger. It does not just affect people elsewhere in the world, or people somewhere in time in the distant future. We all are, in fact, the main character of the climate change story.

C M Shun and S M Lee


[1] IPCC Press Release - Concluding instalment of the Fifth Assessment Report

[2] Global Analysis - 2014 year-to-date temperatures versus previous years

[3] U.S.-China Joint Announcement on Climate Change

  • Elevated ambient radiation level during a rain episode

  • Monday, 1st December 2014

On the night of 22 October 2014, the ambient gamma radiation levels at the radiation monitoring stations in Yuen Ng Fan and King's Park rose significantly, with hourly average dose rates of 0.266 µSv/h and 0.263 µSv/h respectively (Figure 1). The reading at Yuen Ng Fan was more than double its annual average of 0.116 µSv/h in 2013. The one-minute average dose rates at Yuen Ng Fan and King's Park even rose to 0.328 µSv/h and 0.304 µSv/h respectively. Fluctuations in readings were also observed at some other radiation monitoring stations. In the following, we will explain how the precipitation that night would lead to the elevated ambient gamma radiation levels.

Figure 1

Figure 1      Time series of the hourly mean ambient gamma dose rate at Yuen Ng Fan and King's Park; and the
corresponding hourly rainfall from 22 to 23 October 2014.

Humans are continuously exposed to natural radiation in the environment that comes from naturally occurring radioactive materials in our habitats, buildings and structures, as well as cosmic rays from outer space. There are differences in the ambient radiation levels at different parts of the world mainly due to variations in the geographical location and the radioactive contents of local rock and soil. The ambient radiation level also varies under different weather conditions.

On the night of 22 October 2014, localized heavy rain affected Sai Kung, central Kowloon and the western part of Hong Kong Island. Figure 2 shows the radar pictures at 10 p.m. and 11 p.m. that night, indicating a northeast-southwest oriented rain band was affecting the above-mentioned areas. It is observed that the timing for the rise in ambient gamma radiation level readings at Yuen Ng Fan and King's Park were well in line with the rain episode over these areas (Figure 1). The downpour brought natural airborne radionuclides aloft down to the ground, leading to an increase in ambient radiation levels [1, 2].

Figure 2

Figure 2      Radar imageries at 10 p.m. and 11 p.m. on 22 October 2014, showing rain was affecting
Sai Kung and the urban areas.

Furthermore, the prevailing wind condition also helped contribute to the relatively high ambient radiation levels during the episode. On 22 October 2014, southern China was dominated by a continental northeasterly airstream. The localized heavy rain was associated with the convergence between the northeasterlies and the easterlies which set in along the coast that night (Figure 3). Convergence of air masses also facilitated accumulation of natural radionuclides in the atmosphere, which were then brought to the ground by the downpour [3].

Figure 3

Figure 3      Local winds on the night of 22 October 2014, showing the convergence of northeasterlies and easterlies along
the northeastern part of the New Territories and the urban areas.

To assess whether the rise in ambient gamma dose rate could be due to non-natural causes, the rain water samples collected at King's Park during the rain episode on 22 October 2014 were analyzed and see whether artificial radionuclides could be identified. Measurement results show that no artificial radionuclide was detected in the samples. This well demonstrates that the variation of ambient gamma dose rate in this episode is due to changes in local weather.

W.H. Leung and Olivia S.M. Lee


[1] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2000 Report, Volume I, Sources and Effects of Ionizing Radiation, Annex B: Exposures from natural radiation sources.

[2] Wong, M.C., H.Y. Mok, H.K. Lam, 1996: Effects of weather on the ambient gamma radiation levels in Hong Kong, Proceedings of the International Congress on Radiation Protection 1996, Volume 2, p.181-183, International Radiation Protection Association.

[3] Chan, S.W., C.W. Lee, K.C. Tsui, 2010: Atmospheric Radon in Hong Kong, Journal of Environmental Radioactivity 101 (2010) 494-503.

Last revision date: <23 Jan 2015>