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. is only applicable when the actual air temperature is 10 degrees Celsius or below, while in Canada 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.
 "NWS Winter Storm Safety: Windchill Information and Chart" (http://www.nws.noaa.gov/om/winter/windchill.shtml), NOAA National Weather Service
 "Canada's Wind Chill Index" (http://www.ec.gc.ca/meteo-weather/default.asp?lang=En&n=5FBF816A-1), Environment Canada