Hong Kong is a densely populated coastal city with complex topography and large variations in the extent of urbanization. Weather data from various automatic weather stations of Hong Kong reveal that there are significant temperature variations over different parts of the territory. In some cases, there could also be noticeable temperature differences between stations within the same region (e.g. urban area) due to the influence of weather condition and surrounding environment.
Main causes of spatial temperature variations
Among the weather stations in Hong Kong, the Hong Kong Observatory Headquarters (HKOHq) at the heart of Tsimshatsui is one of the World Meteorological Organization stations in Asia with over a century of continuous observations of essential surface meteorological observations. With the advent of the automatic weather station network since 1980s, local climate statistics at enhanced temporal and spatial resolution have also been compiled over the past several decades. Figures 1(a) and 1(b) show respectively the spatial variations of average maximum temperature in summer (June - August) and average minimum temperature of different stations in winter (December - February) in the years 2010-2015. The observed spatial temperature variations can be attributed to three main factors, namely geographic location, site environment, and weather condition (see table in the Appendix). The combined effect of these factors will contribute to the day-to-day or seasonal changes in spatial temperature patterns over Hong Kong.
Figure 1(a) Spatial variation of the average maximum temperature at different stations in summer (June - August) for 2010-2015. Stations on high ground are in green.
Figure 1(b) Spatial variation of the average minimum temperature at different stations in winter (December - February) for 2010-2015. Stations on high ground are in green.
Some interesting cases for illustration
High temperatures on 24-27 June 2016
Under the dominance of the subtropical ridge, the weather was generally fine and very hot in Hong Kong with the maximum temperature at HKOHq topping 35oC on 24-27 June. Located roughly at the centre of the urban area in Kowloon with significant on-going urbanization effect, HKOHq experienced a long-term decreasing trend in wind speed. With very little wind to speak of and under prolonged sunshine during the day, HKOHq reported very high daytime temperatures over this period. Meanwhile, there were also isolated showers each day affecting the northern part of the New Territories and parts of Sai Kung and Hong Kong Island that suppressed daytime temperature rise elsewhere. As a result of site environment, geographic exposure and local weather conditions, the maximum temperatures at HKOHq ended up higher than many stations in the territory (the situation on 25 June as an example shown in Figure 2).
Figure 2 Daily maximum temperature at different stations and daily rainfall distribution in Hong Kong on 25 June 2016. Stations on high ground are in green.
Clear and cold morning on 18 December 2010
Under the influence of a continental airstream over southern China, local weather was mainly fine on 17 December. Light wind condition and clear sky at night enhanced the radiation cooling effect in Ta Kwu Ling, bringing the temperature all the way down to a minimum of 0.2oC in the early morning on 18 December 2010. In the urban area, shielded by the urbanization effect, the night time cooling rate was much slower and the minimum temperature at HKOHq was 10.7oC, more than 10 degrees higher than that of Ta Kwu Ling (Figure 3).
Figure 3 Schematic showing variation of daily minimum temperatures across the territory from urban to rural areas on 18 December 2010.
Consistency in temperature observation amid changes
Hong Kong is a high-density city with a constant need for new development. HKOHq and other weather stations, either in rural or urban areas, will unavoidably experience a varying degree of changes in their surrounding environment over time (e.g. land use, building development, vegetation coverage, etc.). The temperature at HKOHq has been serving as one of the key references for Hong Kong since 1884. The setting up of more weather stations over different parts of Hong Kong, including those within the urban area itself, has added more spatial information for reference in recent years. Analyses of the temperatures of HKOHq and other urban stations nearby suggest that the year-to-year and seasonal variations of these urban stations, as well as their overall trends, are generally in line with each other despite the observed spatial differences in geographic location and site environment. Long-term weather observations from these stations, in particular data at HKOHq that are more than a century long, document the climate variations in Hong Kong through time due to global climate change impact as well as local urbanization effect, thereby providing an invaluable source of climate information not only for Hong Kong but also for the world.
T.C. Lee, K.W. Li and Y.H. Lau
 秋風夜雨 (Chinese only)
 Why is my home colder?
 The Higher the Place, the Stronger the Wind?
 What is gravity wind?
 Warm nights
 Example: Exposure of Kai Tak Wind Station
Over the past century, Hong Kong has experienced a number of severe rainstorms. Previously, we have talked about the phenomenal rainstorm on 19 July 1926 of which daily rainfall of 534.1 mm were recorded at the Hong Kong Observatory. However, there is a fiercer storm - the great rainstorm of the century on 29-30 May 1889 which beats the 1926 rainstorm in rainfall records with the total rainfall reaching 697.1 mm in 24 hours, amounting to one third of the yearly rainfall!
According to the weather log of the Hong Kong Observatory[2, 3], this great rainstorm of the century started in the small hours of 29 May 1889 with continuous thunderstorms moving from southwest to northeast affecting Hong Kong. After a lull period between 2 and 3 p.m., moderate rain returned and lasted till midnight. Then another batch of severe thunderstorms raged across the territory between 1 and 5 a.m. on 30 May. Besides torrential rain, lightning flashed incessantly and thunder roared throughout the night. While the downpour became less intense after 6 a.m., the rain did not stop until late afternoon that day.
The rainstorm event remains the keeper of many rainfall records at the Hong Kong Observatory, including running 3-hour rainfall, running 4-hour rainfall, running 24-hour rainfall, and 2-day to 7-day rainfall (please see Table 1 for details). Considering the 30-year average annual rainfall at the Observatory at the time (i.e. 1884-1913) of 2113 mm, this great rainstorm of the century essentially brought one third of the yearly rainfall just within 24 hours! This is also a record in itself.
According to the comprehensive report on the rainstorm compiled by Mr. Samuel Brown, the Surveyor General (later Director of Public Works) at the time, the rainfall over Hong Kong Island, in particular on the hill slopes, was probably even higher than that of the Observatory in Kowloon. Unfortunately, the raingauge at the Victoria Peak overflowed during the heavy downpour and only an estimated 24-hour rainfall amount of about 707 mm (ending at 10 a.m. on 30 May) was documented in the Observatory's publication.
Figure 1 Hourly rainfall recorded at the Hong Kong Observatory from 28 to 30 May 1889.
Table 1 Maximum rainfall of different durations recorded at the Hong Kong Observatory for the May 1889 rainstorm.
Causes of the Rainstorm
Since this rainstorm happened a long time ago during the late Qing dynasty, only limited weather observations, mostly from the Observatory headquarters at Tsim Sha Tsui, Kowloon, were available for analysis of the weather pattern at the time. The daily mean pressure at the Observatory fell gradually from 1007.9 hPa on 26 May to 1003.2 hPa on 30 May. A re-analysis of the surface pressure pattern revealed a broad trough of low pressure over central and southern China, with rather tight pressure gradient along the coastal areas on both 29 and 30 May. Winds at the Observatory veered from easterly to southwesterly on 29 May when the heavy rain began. Fresh to strong southwesterly winds were also reported at the Victoria Peak on 29-30 May. In the absence of any reported tropical cyclone activity, the unsettled weather could be attributed to the broad trough, with the enhanced southwest monsoon bringing an ample supply of moisture that sustained the development of the rainstorm over the south China coastal areas.
Figure 2 Re-analysis of the surface pressure on 30 May 1889 (source: National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Physical Sciences Division).
Figure 3 Daily mean wind speed and direction at the Hong Kong Observatory from 26 to 31 May 1889.
Damages and Casualties
According to the newspapers and the Surveyor General's report, the severe rainstorm on 29-30 May 1889 was one of the most destructive rainstorms in the history of Hong Kong with numerous reports of flooding and landslides. Traffic in the city, telegraph communications and water supply from Tai Tam Reservoir (called Tytam reservoir at the time) were interrupted. Consequently, some people were obliged to use muddy water that was flowing down the side channels and the streams as their source of water supply, bringing serious impacts to the society and people's livelihood.
Over Hong Kong Island, torrents of rain water mixed with debris severely damaged the Glenealy Nullah and the Albany Nullah located at the mid-levels. For Albany Nullah (starting around point A in Figure 4), the Tytam service tank and filter beds above the Bowen and Garden Roads (point C in Figure 4; photo in Figure 5) were severely damaged by landslides from the steep slope (photo in Figure 6) immediately below the Rural Building Lot No. 7 (point D in Figure 4) and filled up by rock and debris on 29 May. This led to even more serious landslides and heavy rush of water when yet heavier rain fell during the night and early morning of 30 May. An estimated 13,800 cubic metres of earth was washed away from the slopes of the service tank and filter beds while another equal quantity was probably washed away from the banks of the nullah below the service tank. As a result, part of the tramway connecting the city and the Peak together with two bridges were swept away. The torrents wreaked havoc at the Murray Barracks (around point E in Figure 4, photo in Figure 7) which was flooded to around 0.6 -1.2 metre where order was given to evacuate the quartered servicemen and stores from the basement floors.
Figure 4 Diagram showing the locations of the major damages at the Glenealy Nullah, Albany Nullah, Tytam Aqueduct (along Bowen Road) and Happy Valley.
Figure 5 Damage to the Tytam service tank (source: National Archives, UK).
Figure 6 Landslides above the Tytam service tank (source: National Archives, UK).
Figure 7 Neighbourhood of Murray Barracks after the rainstorm (source: CM Shun).
Over the Glenealy Nullah (starting near point B in Figure 4, photo in Figure 8) located about 640 metres to the west of the Albany Nullah, the amount of water running down the ravine was considerably augmented due to the ongoing building construction work uphill (near point F in Figure 4). The floods and landslides at the mid-levels and downstream areas deposited large amount of debris in Wyndham Street (photo in Figure 9), Queen's Road Central and Pedder Street (around point G in Figure 4; photo in Figure 10). At Zetland Street (Figure 11), the torrents flooded the street to around 1.5-1.8 metre, tore up the roadway and ploughed up the bottom to a depth of 1.5-1.8 metre.
Figure 8 Damage to the Glenealy Nullah (source: National Archives, UK).
Figure 9 Debris on Wyndham Street in Central after the rainstorm (source: National Archives, UK).
Figure 10 Pedder Street in Central after the rainstorm (source: National Archives, UK).
Figure 11 An artist's impression of the torrents at Zetland Street, Central (source: CM Shun).
Serious landslides also occurred along the steep slopes above the Tytam Aqueduct along Bowen Road where the masonry of the aqueduct was damaged in three places (near points H, I and J in Figure 4), involving some 23,000 cubic metres of debris. One of these slides involving 3,800 cubic metres of debris swept through the Hong Kong Cemetery all the way onto the racecourse in Happy Valley (around point K in Figure 4). Landslides also affected parts of Wanchai, Western District and Kennedy Town. The Surveyor General estimated that some hundreds of landslides of various sizes had probably occurred over Hong Kong in this great rainstorm of the century.
Casualties and Remedial Works
It was estimated that a total of 27 people were killed, including six coolies killed by lightning at the Peak, and another 17 were missing[4, 7]. The remedial work were mostly handled by Mr. S. Brown (the Surveyor General), Mr. F. A. Cooper (Acting Assistant Surveyor General), and Lieut. Colonel H. Champernowne (Royal Engineer). The cost of damage to government property was estimated at a total amount of $112,783 at the time which amounted to about 6% of the annual expenditure of the Government in 1889[4, 8].
As water supply from Tai Tam Reservoir was interrupted, people had to rely on the Pokfulam Reservoir supply. However, as muds went into the reservoir, the quality of water became a major concern. Water quality tests were immediately done. Water supply thus became the top priority of the Surveyor General. Thus, one of the major outcomes of the 1889 rainstorm was the establishment of the Water and Drainage Department in 1890 to handle drinking and foul water.
The rainfall records set by this great rainstorm of the 19th Century and the associated damages were indeed rather alarming. A number of the associated rainfall records have yet to be broken. Under the trend of the intensifying climate change, extreme weather events including rainstorms are becoming more frequent. Past rainfall records, including the 1-hour and running 2-hour rainfall records, had already been broken in recent years. Recurrence of the great rainstorm of the century will only be a matter of time.
Recently, Mr Wong Hok Ning, Head of the Geotechnical Engineering Office, who is about to retire, also described landslip as a "sleeping lion" which seemed to be waking up, and even though the risk of landslide was in general higher for man-made slopes, natural slopes would have greater response when the rainfall reached a certain level, leading to landslides. Recalling that the Surveyor General estimated that some hundreds of landslides of various sizes had probably occurred over Hong Kong in the great rainstorm of 1889, are we fully prepared if this or an even greater rainstorm were to recur in the foreseeable future? Facing the increasing risks brought by climate change, indeed we will need to increase our alertness and preparedness, and to enhance public education so that Hong Kong will be resilient and climate ready.
T.C. Lee, C.M. Shun and K.Y. Ma*
(* Mr K.Y. Ma is a retired government engineer and also an enthusiast in the history of engineering in Hong Kong. He is currently Adjunct Associate Professor at the Department of Real Estate and Construction, University of Hong Kong)
 The Phenomenal Rainstorm in 1926 http://www.hko.gov.hk/blog/en/archives/00000135.htm
 Hong Kong Observatory, 1890: Observations made at the Hong Kong Observatory in the year 1889.
 Chan, C. W., 1976: The rainstorms of May 1889 and July 1926, Royal Observatory Occasional Paper No. 33.
 S Brown, 1889: Report on great storm of 29th and 30th May 1889.
 20th Century Re-analysis (V2), National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Physical Sciences Division http://www.esrl.noaa.gov/psd/data/composites/subdaily_20thc/index.html
 "After the great storm", China Mail, 31 May 1889.
 Ho Pui-yin, 2003: "Weathering the Storm: Hong Kong Observatory and Social Development", Hong Kong University Press, 364 pp.
 Hong Kong Government, 1890: Report on the blue book and departmental reports for 1889.
 Hong Kong Telegraph, 30 May 1889.
 China Mail, 1 June 1889.
 Hong Kong Government Gazette No. 215 of 1890.
 Sing Tao Daily, 19 December 2016 (in Chinese)
While there are 12 to 13 full moons every year, only those on the Mid-autumn Festivals and those coincide with lunar eclipse would attract more public attention. With the growing popularity of astronomical observations and the wide coverage by the media and on social networking platform, the big 'Super Moon' has recently become a hot topic in town.
The biggest 'Super Moon' this year (2016) will occur on the night of 14 November. "How does this 'Super Moon' compare with those in the past?" and "When will a 'Super Moon' be bigger than this one?" are the common questions in mind.
Let's have an eyesight test - Can you identify those 'Super Moons' bigger than the one on 14 November 2016 from the following figure showing the top ten 'Super Moons' between 1901 and 2050?
Here is the answer: The 'Super Moons' in 1912, 1930, 1948 and 2034 are bigger than the one on 14 November 2016, i.e. the 'Super Moon' this year (2016) will be the biggest since 1948 and a bigger one will not occur until 2034.
In fact, using the 'distance between the moon and the earth' as a parameter to compare the size of 'Super Moons', the biggest 100 'Super Moons' between 1901 and 2050 range only between 356,375 km and 357,099 km, whereas using the 'angular diameter' as a parameter, they range only between 0.557o and 0.559o. The difference is so small that it is hardly distinguishable by the naked eyes and even with ordinary instruments.
Then, is there any difference between the 'Super Moon' on 14 November 2016 and an ordinary full moon in the same year?
The table below compares the 'Super Moon' on 14 November 2016 and the 'average value of full moons' in 2016 . One can derive from the 'distance between the moon and the earth' and the 'angular diameter' that the size of the 'Super Moon' will appear around 7 per cent larger than an ordinary full moon.
The figure below shows the visual difference between the 'Super Moon' on 14 November 2016 (right) and an ordinary full moon (left).
With only one moon in sight on 14 November 2016, can you really distinguish the difference between the 'Super Moon' and an ordinary full moon?
Astronomy unit of the Hong Kong Observatory
(H.Y. Mok, David Hui, Otto Cheng, K.C. Fung, W.K. Wong and S.C. Chee)
 'Super Moon' is loosely defined as a full moon near the perigee.
 'Angular diameter' is an angular measurement describing how large a sphere or circle appears from a given point of view.
 'Distance between the moon and the earth' is measured from their centre of mass.
 'Average value of full moons' is computed based on all full moons in the year.
'Cool Met Stuff' - Mid-Autumn 'Super Moon' (Chinese only): http://www.youtube.com/watch?v=Rgeh-u6wBwA
'Cool Met Stuff' - Moon Illusion (Chinese only): http://www.youtube.com/watch?v=tlqk4J4tFN8