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Friday, 21st January 2011

What is a normal year?

On the surface, the weather in 2010 can be said to be normal for Hong Kong. The mean temperature was 23.2 degrees, just 0.1 degree above normal, while the total rainfall was 2371.7 millimetres (mm), differing by only 11 mm from the annual mean.

Deep down, it is a different story. The weather in January and February was unusually warm, bringing early blossoms. However, the low temperatures recorded in March, June, October, November and December all came within the ten lowest respectively for those months (since records began in 1884).

In terms of rainfall, a few heavy rain days in the year were enough to bring a quarter of the yearly total. This included two heavy rain days in July that necessitated the issuing of the Black Rainstorm Warning, plus a couple of days of downpour brought by tropical cyclones Lionrock and Fanapi in September.

All these mean that 2010 was less like a normal year than one of extreme weather.

What about the longer term? If we look at the past 120 years in Hong Kong, here is what my colleagues found in their statistical analysis of changes in extreme weather. In 1900, a 100-mm rain in an hour would occur on average once every 37 years (i.e. a return period of 37 years). By 2000, the return period had decreased to 19 years, mean that such heavy rain has become twice as frequent.

Figure 1 presents the highest hourly rainfall at the Observatory Headquarters. It shows that for the 80 years from the 1880s to 1960s, it took 40 years, until 1926, for the hourly high to break the 100-mm mark. Another 40 years passed before the record was exceeded in 1966. From then on, it was broken three more times, each time more rapidly than the previous one, until 7 June 2008 when the new record of 145.5 mm was set, breaking the previous one by a wide margin of 30 mm.

Figure 1     Record hourly rainfall record at the Hong Kong Observatory Headquarters, 1884-1939 and 1947-2010.
Figure 1      Record hourly rainfall record
at the Hong Kong Observatory Headquarters, 1884-1939 and 1947-2010.


In respect of the temperature, back in 1900 the return period for a high of 35 degrees was 34 years. By 2000 it had decreased to 5 years, meaning that such hot weather had become 7 times more often. On the other hand, the return period for a 4-degree day in 1900 was 6 years. By 2000, however, the estimate is that it would take 150 years for it to re-appear (the last time the Observatory registered 4 degrees or lower was in 1972). All these mean that high temperatures have become more frequent, and low temperatures rarer.

We can also look at pre-war and post-war temperatures separately, as the 1950s marked the start of rapid development in Hong Kong. Figure 2 shows the distribution of winter daily minimum temperatures for the two periods. It can easily be seen that what looks like a 'normal' curve for the pre-war period has become skewed towards higher temperatures for the post-war period. The same skewness can also be observed in the picture for hot-seasons highs, Figure 3. Hong Kong's heading towards higher temperatures is unmistakable. This trend has been reported in many other places in the world also.

Figure 2     Frequency distribution of daily minimum temperatures in winter months (December to February) in the pre-war (1884-1939) and post-war period (1947-2010). (There were no complete records in 1940-1946.)
Figure 2      Frequency distribution of daily minimum temperatures in winter months
(December to February) in the pre-war (1884-1939) and post-war period (1947-2010).
(There were no complete records in 1940-1946.)



Figure 3     Frequency distribution of daily maximum temperatures in the hot season (May to September) in the pre-war (1884-1939) and post-war period (1947-2010).
Figure 3      Frequency distribution of daily maximum temperatures in the hot season
(May to September) in the pre-war (1884-1939) and post-war period (1947-2010).


Global warming is already happening. If we do nothing about it, our future generations will certainly be at stake. We only have ourselves to blame.



B.Y. Lee and H.Y. Mok



Last revision date: <17 Jan 2013>