A couple of days ago (22 May 2013), the Hong Kong Observatory issued the Black Rainstorm Warning Signal (the so-called 'Black' rainstorm locally). This was the first 'Black' rainstorm since July 2010. It was also the first one in 13 years (13 years once again ) that occurred during the morning rush hours when people normally go to work or school. Naturally it drew much public attention, in particular on how long the 'Black' rainstorm would last and whether one would need to go to work or school.
The existing Rainstorm Warning System by the Observatory has been in operation since 1998. The 'Amber', 'Red' and 'Black' rainstorm respectively represent heavy rain has fallen or is expected to fall generally over Hong Kong, exceeding 30, 50 and 70 millimetres in an hour, and is likely to continue. The Rainstorm Warning System operates on the basis of objective criteria and scientific approach. The rainfall values mentioned above are easily understood and effective, but they are not rigid mandates. When prolonged rain occurs, the warning signals may be issued even if those rainfall criteria are not reached for individual hours. By the same token, a temporary easing off of heavy rain while the weather remains unsettled would not warrant an instant change of the warning signal already in force. This avoids frequent changes in the warning status from 'up and down, down and up', which might cause more confusion. As for other weather warnings, public safety is always the primary consideration and the most important factor in the decision of issuance or cancellation of rainstorm warning signals
The formation, growth and decay of rainstorm can be very rapid and the variations rather substantial. All these changes can happen within just a few hours. Precise forecasting of rainstorms remains a major challenge and goal of the Observatory and the meteorological community worldwide. Since the state-of-the-art technology still cannot fully grasp the future evolution of rainstorms, it is yet hard to accurately predict the issuance or cancellation of a rainstorm warning signal.
Rainstorms can bring about severe traffic disruption, floods, landslips and even casualties. To safeguard the public, forecaster will continuously monitor the latest weather situation and provide updated information to the public. During heavy rain episodes, please stay tuned to the latest weather information and warning bulletins of the Observatory via radio, TV, Observatory's website (www.hko.gov.hk), 'MyObservatory' mobile app or 'Dial-a-weather 187 8200'. Remain on the alert against casualties and losses.
Rainstorms are inevitable. The prompt and orderly responses of the public and the society in taking the necessary precautions do not come easily. These were achieved through feedbacks from members of the public, dedicated efforts of various departments and organizations as well as educating the youths at primary/secondary schools over the years. The Observatory will continue to enhance our weather services, in particular the contents and dissemination of information, with a view to facilitating the public to clearly comprehend the latest weather situation.
 The last two 'Typhoon Signal No.10' also separated by 13 years, i.e. 1999 and 2012.
 Refers to Monday - Friday, non-public holidays. The previous Black Rainstorm Warning Signal that happened during busy morning hours was on 12 June 2000 (Monday).
 Please refer to the link http://www.hko.gov.hk/wservice/warning/rainstor.htm for more details on the Rainstorm Warning System.
Thursday, 9th May 2013
The "Geng-Zi" typhoon disaster in 1900
While about 90 percent of the tropical cyclones affecting Hong Kong occur between June and October, tropical cyclone warnings have been issued in November or even into early December due to the approach of some late-season "autumnal typhoons". Typhoons approaching the South China coast late in the year are often weakened by the northeast monsoon. However, in the early hours of 10 November 1900, Hong Kong was caught off guard by the fierce winds of a typhoon. Since the year 1900 was the "Geng-Zi" year in the Chinese calendar, this late-autumn typhoon event was called the "Geng-Zi typhoon disaster ( 庚子風災 )". Despite being one of the deadliest in Hong Kong history, this typhoon event is less known to many people when compared with the other major pre-World War II typhoon disasters in 1874, 1906, and 1937. Therefore, we have collated information from a number of government reports, newspapers and historical meteorological records published around that time to unveil further details about this autumnal typhoon disaster[1-6].
The ignored typhoon signals
The typhoon originated over the sea to the east of the Philippines on 5 November 1900. It tracked northwestward in the next couple of days and entered the South China Sea on 7 November. It then gradually turned northward edging towards the coast of Guangdong during 8 and 9 November (Figure 1). Based on the weather map at the time and the modern re-analysis, this typhoon appeared to be relatively large in size with a circulation of around 800 to 1000 kilometres in diameter (Figures 2 and 3). As such, both the Manila Observatory and Hong Kong Observatory had given due warnings to the public of the existence of this typhoon over the South China Sea. In Hong Kong, typhoon signals were hoisted starting from 8 November to warn the public of an approaching typhoon . By 6:15 p.m. on 9 November, typhoon gun was fired to warn the public that gale force winds were expected in Hong Kong.
Figure 1 Track of the centre of the "Geng-Zi" Typhoon from 5 to 10 November 1900.
Figure 3 Re-analysis of mean sea level pressure and 10-min Wind on 9 November 1900. (Source : NOAA 20th Century "Reanalysis" ).
With the typhoon approaching, the northeasterly winds in Hong Kong strengthened and reached gale force on the night of 9 November 1900. By 4 a.m. on 10 November, winds reached storm force at the Observatory as the eyewall of the typhoon approached from the south. By 5:00 a.m., the typhoon was at its height in the territory when the hourly mean wind speed at the Observatory reached about 61 knots (113 km/hr) (Figure 4). The center of the typhoon very likely passed over the eastern part of Hong Kong in the following 3 hours as the barometer at the Observatory fell to a minimum of 974.9 hPa at around 6:00 a.m. (Figure 5). This still stands as the lowest sea-level pressure record for November to date.
Figure 4 Hourly mean wind speed and direction at the Hong Kong Observatory Headquarters (HKO Hq) on 9-10 November 1900. The wind speeds were calculated from data recorded by the Beckley anemometer at HKO Hq in 1900 and the conversion factors from HKO Technical Note No. 66.
Figure 5 Hourly mean sea level pressure recorded at HKO Hq from 9 to 10 November 1900.
Given the hourly observations taken at the Observatory through the typhoon event, we have made an attempt to estimate the possible hourly positions as well as the pressure pattern of the typhoon during its passage over Hong Kong. The results are depicted via an animation given here (http://www.hko.gov.hk/img/1900typ_obs_isobar_track.GIF). Note that the isobars shown in the animation were constructed based on the assumptions that the typhoon remained generally circular in shape and its intensity unchanged prior to landfall.
As per the reports from the Governor and newspapers[1, 4], the Observatory had given due notice of this impending typhoon to the public and the inclement weather also fully validated the Observatory's prediction. Regrettably, the public gave less attention to the warnings and little precautions were taken as many of them disbelieved that such a violent storm would affect the territory in this season.
How rare is this November typhoon?
As mentioned earlier, the maximum mean hourly wind speed at the Observatory reached 61 knots at 5 a.m. on 10 November. This value is comparable to the records of some of the well-known Signal No. 10 typhoons, such as Gloria (1957), Ruby (1964) and Rose (1971). In the 130-year record of the Observatory, this Geng-Zi typhoon was the first on record that had made landfall in Hong Kong in the month of November. The second tropical cyclone that had ever made landfall in Hong Kong in November occurred in 1939. This typhoon moved in a rare west to east direction across Hong Kong on 23 November 1939. Signal No. 9 was hoisted during the height of the storm. Although the Hurricane Signal No. 10 did not exist in 1900, and had never been hoisted in November since its inception in 1931, the wind and pressure records at the Observatory strongly suggested that the Geng-Zi typhoon could have brought hurricane force winds in Hong Kong, especially over the eastern part.
Besides the 1900 and 1939 typhoons, there was only one more typhoon (1915) that had brought gale force winds at the Observatory during November before World War II. After World War II, there were two typhoons which necessitated the hoisting of Signal No. 8 or higher in November, namely the typhoons in 1954 (Signal No. 9) and 1972 (Signal No. 8). Coincidently, both of them were named Pamela. Counting all of the above-mentioned typhoons, Signal No. 8 or higher event in November occurred on an average of about every 26 years in the past 130 years in Hong Kong. Thus, statistically speaking, Hong Kong may be already due for the arrival of a late-autumn Signal No. 8 typhoon. The last time that tropical cyclone signal No. 3 or above were hoisted in the month of November due to a typhoon was already back in 1993 during the approach of Typhoon Ira. Although only Signal No. 3 was hoisted for Ira, a brief period of gale force winds was experienced in parts of the territory.
Damages and casualties
The "Geng-Zi" Typhoon caused extensive damages and heavy casualties to the territory (Figure 6). Numerous sampans and boats were sunk or even smashed to matchwood by the raging waves. Ten steam-launches and over 110 junks were also sunk and the harbour was full of wreckage. The gunboat H.M.S Sandpiper sank at her mooring and her crews, with one exception, were gallantly rescued by the torpedo-destroyer H.S.M. Otter. The large dredger "Canton River" was blown over and sunk. The Star Ferry pier was also heavily damaged. Over land, there were many reports of damages to houses, especially in the Peak District. All matsheds erected on the reclamation land over Yaumati were leveled to ground by the high winds. Many trees were damaged or uprooted. Lamp posts and telephone posts were bent at all angles by the furious winds. Over 200 lives were lost in these few fatal hours.
Figure 6 Damages in Hong Kong brought by the "Geng-Zi" Typhoon as reported by the Illustrated London News on 22 December 1900.
Judging from the meteorological records and the historical document descriptions, this typhoon brought fearful havoc and desolation to the ill-prepared society in Hong Kong in 1900. This event highlights that, besides timely and accurate forecasts, the awareness and preparedness of the public to respond are also key factors of an effective typhoon warning system. No matter in peak or off-peak seasons, we should always remain vigilant to the typhoon threat in Hong Kong.
C.M. Shun(1), K.Y. Kong(2), and T.C. Lee(1)
(1) Hong Kong Observatory (2) Weather Prediction Center - NOAA, U.S.A.
 G.S.P. Heywood (1940). "The Typhoon of November 17th to 25th, 1939". Appendix B of the Meteorological Results 1939, Royal Observatory
Friday, 3rd May 2013
Which day is the Tuen Ng Festival for this year (2013)?
Which day in the Gregorian calendar is the first day of the 5th lunar month this year (2013)? This question has aroused public concern and discussion (http://www.ettoday.net/news/20121120/129516.htm). According to the "Hong Kong Observatory (HKO) Almanac 2013" [Figure 1], using the high-precision astronomical data of the HM Nautical Almanac Office, U.K. and the U.S. Navy Observatory, the first day of the 5th lunar month in 2013 will be on 8 June in the Gregorian calendar, same as that of the official almanac of Mainland China. However, some common Chinese folk calendars fix that day to be 9 June. Because of this difference, there appears to be two different Gregorian dates of the Tuen Ng Festival (the 5th day of the 5th lunar month) in 2013.
In fact, such discrepancy is not uncommon. Table 1 shows that there will be three more similar situations in the next 20 years. In particular, there are two different dates for the Lunar New Year Day in 2030!
Table 1 Comparison of Gregorian-Lunar calendar conversion of HKO Almanac and some Chinese folk calendars.
What are the reasons behind the discrepancies? Let us first understand how the first day of a lunar month is determined from the astronomical point of view.
Each month in the lunar calendar starts with a "New Moon", which occurs when the Moon and the Sun move to the same longitude on the ecliptic, . Different computed dates in different calendars come about due to two main reasons: (a) outdated astronomical computation; and (b) change in the standard time adopted.
First of all, astronomical computation has become more and more precise with the advance in science and technology. Official authorities will make use of the latest astronomical data and computation methodology to re-calculate and revise the calendars from time to time. The Hong Kong Observatory and the Zijinshan Astronomical Observatory of Mainland China will revise the official calendars whenever there are updated information and decisions issued by the International Astronomical Union and the Jet Propulsion Laboratory in the US. On the other hand, some of the Chinese folk calendars are compiled based on outdated astronomical data and methodology or duplicating the data from old versions of official calendars. In case when the "New Moon" occurs very close to midnight, it is possible to have different start times for the first day of lunar month based on different astronomical data and computation methodologies. For example, the "New Moon" of the 5th lunar month this year (2013) will occur at 23:56 Hong Kong Standard Time (HKT) on 8 June based on the latest astronomical data and computation methodology. If outdated data and computation methodology are used, the "New Moon" may appear after 00:00 (HKT) on 9 June and the first day of the 5th lunar month will then occur on 9 June. Table 2 shows that all the times for the "New Moons" of the four dates mentioned in Table 1 will occur near midnight based on the latest astronomical data and computation methodology.
Table 2 The Gregorian calendar dates and Hong Kong Standard Times of the New Moons for the first days of the lunar months listed in Table 1.
Secondly, even though the same astronomical data and computation methodology are used, the results may still be different if different standard times are adopted. In late Qing and early Republic period, the 116.383 degree East (longitude of Beijing) of apparent solar time was used as time standard . However, a unified time zone, which is 120 degree East of longitude of apparent solar time, was adopted as the standard time for Mainland China soon after the establishment of the People's Republic of China. The times for "New Moons" and the "Solar Terms" of the calendar was then converted to the Coordinated Universal Time (UTC) plus eight hours. It is noted that the difference in these two standard times adopted may lead to a difference of about 14.468 minutes for defining the beginning of a day. Therefore, adopting different standard times could generate different results in compiling the calendar even though the same astronomical data and computation methodology are used. For example, the first day of the 1st lunar month in 1916 would be on 4 February (the "New Moon" occurred at 00:05 on 4 February) if the current standard time is used, but it was fixed on 3 February (the "New Moon" occurred at 23:51 on 3 February) based on the standard time used at that time.
There are many publications of Chinese folk calendars in the market. Most of them have not specified the authors and source of data. It is therefore rather difficult to trace the source of astronomical data and computation methodology as well as the standard time adopted. As a difference in one minute may cause the difference of one day in Gregorian-Lunar calendar conversion, it is not surprising to find discrepancies between the latest official calendars and folk calendars. Please double-check the Observatory's Almanac if in doubt (http://www.hko.gov.hk/gts/astron2013/almanac2013_index_e.htm) [Figure 2].