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  • "Parade of the Five Planets" in Celebration of the Chinese New Year

  • Thursday, 4th February 2016

Venus, Jupiter, Mercury, Mars and Saturn are the five planets that can be observed directly with unaided eyes. They have appeared together in the sky since mid-January this year and, weather permitting, will remain observable in Hong Kong for a few weeks. The astronomical phenomenon, known as the "Parade of the Five Planets", will still be there to greet you in celebration of the Chinese New Year - just look towards the southeast and southwest before dawn(Figures 1 and 2).


Figure 1

Figure 1      Schematic diagram showing the positions of the "Parade of the Five Planets" at 6:00 am on 8 February 2016 (the First Day of Chinese New Year).



Figure 2

Figure 2      Azimuth and altitude of the five planets at 6:00 am on 8 February 2016 (the First Day of Chinese New Year).



As Mercury can only be seen for a short time after sunset or before sunrise, the opportunity for all five planets coming into view at the same time is rather infrequent. The chart of "Time of Rise and Set of the Sun and Planets" in the Almanac published by the Hong Kong Observatory (Figure 3) is a handy tool to track the occurrence of this phenomenon. As an example, one can deduce from the chart that following the sunset at 18:15 on Chinese New Year Eve (7 February 2016), the five planets in the sequence of Jupiter, Mars, Saturn, Venus and Mercury will appear respectively in the Hong Kong night sky at 20:37 that evening, 00:43,02:42,05:09 and 05:26 after midnight. In other words, the five planets will line up in the sky for all to observe from about half past five in the morning till daybreak on the First Day of Chinese New Year (8 February 2016).


Figure 3

Figure 3      Times of the rising and setting of the Sun and the planets in Hong Kong during 2016. Areas in yellow indicate the periods when all five planets can be observed in the sky after sunset and before sunrise.



Similar phenomenon will occur again between mid-July and early September this year, but the locations of the planets will be different and the observable periods will be in the evening before the setting of the five planets. Those who would like to test their astronomical knowledge are welcome to try out the "Time of Rise and Set of the Sun and Planets" chart to determine the timing of occurrence for the phenomenon (http://www.hko.gov.hk/gts/astron2016/2016rise-set.pdf).



Astronomy unit of the Hong Kong Observatory
(David Hui, Otto Cheng, KC Fung, WK Wong and SC Chee)


Related links:

2016 Almanac (web version): http://www.hko.gov.hk/gts/astron2016/almanac2016_index_e.htm

"Hong Kong Observatory Almanac 2016": http://www.hko.gov.hk/press/D4/2015/pre20151123.htm

Weather Information for Astronomical Observation: http://www.hko.gov.hk/gts/astronomy/astro_portal.html


  • Super Typhoons in 2015

  • Friday, 29th January 2016

Under the effect of climate change, there are changes in the frequency, intensity, spatial extent, duration and timing of extreme weather events. In 2015, besides the local and global record breaking temperatures, tropical cyclone activities in the western North Pacific, especially the occurrences of super typhoons, were also eye-catching news that captured the attention of both the media and the public. For example, the havoc wreaked by Super Typhoon Soudelor in Taiwan and the "twin typhoons" of Goni and Atsani in August 2015 hit the headlines of local as well as overseas newspapers. But are super typhoon activities actually more frequent in the western North Pacific (WNP) basin in 2015 ?

Before turning to tropical cyclone statistics, let us first of all re-visit what is meant by "Super Typhoon". Tropical cyclone intensity is classified in accordance with the World Meteorological Organization's recommendation by the maximum sustained wind speeds near the centre. "Typhoon" in a generic sense represents the most intense cyclones under the classification scheme in WNP[1]. However, meteorological services and warning centres in WNP have since adopted different sub-classification schemes within the "Typhoon" category in defining "Super Typhoon" or similar terms (Table 1)[2]. We need to recognize such differences in the interpretation of "Super Typhoon" statistics compiled and reported from various sources.

Table 1

Table 1      Definition of "Super Typhoon" (or similar terms) among various meteorological
services and warning centres in WNP.


In Hong Kong, a typhoon with maximum sustained winds (10-minute average) of 185 km/hr or above near its centre is classified as a "Super Typhoon". For WNP and the South China Sea in 2015, there were altogether 13 super typhoons according to the definition of the Hong Kong Observatory[3], eight more than the long-term (1961-2010) yearly average of about five and making it the most active super typhoon year since comprehensive record began in 1961.

Tropical cyclone relies on moisture as its energy source and its intensity change is affected by the atmospheric conditions[4] and the underlying sea surface temperature along its path. As such, more super typhoons in 2015 can partly be attributed to the current El Niño event. As previously discussed in the blog article "No Tropical Cyclone Warning Signals in August and September", the above-normal sea surface temperature over the central and eastern equatorial Pacific results in abnormal atmospheric circulation over the Pacific and in turn displaces the breeding ground of tropical cyclones in the WNP further east. Figure 1 shows that the tropical cyclone genesis positions in 2015 were indeed located further east, with a majority of them forming to the east of 140o E, including all 13 super typhoons and two crossing the dateline and entering WNP (hence bearing non-WNP "foreign" names: Halola and Kilo). Moving typically west to northwestwards after genesis, tropical cyclones forming further east will stay over the oceans longer during their lifespan, thereby increasing the chance for them to develop into super typhoons under relatively high sea surface temperature and favourable atmospheric conditions.

Figure 1

Figure 1      Tropical cyclone genesis position in 2015. The shaded area in the background corresponds to
the long term average (1961-2010) of tropical cyclone genesis distribution.





CHOY Chun-wing  WU Man-chi


References:

[1] World Meteorological Organization Technical Document, Tropical Cyclone Programme, Report No. TCP-23, Typhoon Committee Operational Manual, Meteorological Component, 2014 Edition

[2] For details, please refer to the website of ESCAP/WMO Typhoon Committee, http://www.typhooncommittee.org/tropical-cyclone-classification/

[3] These 13 Super Typhoons are Higos, Maysak, Noul, Dolphin, Chan-hom, Nangka, Soudelor, Atsani, Goni, Dujuan, Koppu, Champi and In-fa.

[4] The suitable atmospheric conditions include weak vertical wind shear, sufficient lower levels convergence, upper level divergence and moisture transport, etc.


  • Seasonal Rainfall under El Niño

  • Wednesday, 13th January 2016

The current El Niño event started in May 2014 and persisted for more than 18 months, the longest since 1950 and the strongest since 1997/98.

We pointed out earlier that El Niño's impact on the climate of Hong Kong was mainly manifested in more abundant winter and spring rainfall compared to the ENSO-neutral state[1], with tropical cyclone seasons starting no earlier than June (blog: Anticipated Return of El Niño). The rainfall amounts recorded in winter (Dec - Feb) 2014/15 and spring (Mar - May) 2015 were 118 and 606 mm respectively, both higher than the corresponding average seasonal rainfall under the ENSO-neutral state (Figure 1). The tropical cyclone season of 2015 started in June (Tropical Storm Kujira). All these observations were consistent with the statistical results.

Figure 1

Figure 1      Winter (Dec-Feb) and spring (Mar-May) rainfall of Hong Kong under the ENSO-neutral state (green)
and El Niño (pink) during 1950-2015.


For the winter of 2015/16, the rainfall recorded at the Observatory up to early January 2016 already exceeded 120 mm. With the normal range of winter rainfall lying between 64 and 147 mm, therefore it is certain that this winter will end up with normal to above-normal rainfall, a trend again consistent with the tendency of relatively wet winters under El Niño.

In the aforementioned blog, we also pointed out that El Niño would not necessarily give rise to rainy summers in Hong Kong. In fact, the chance of above-normal rainfall was noticeably lower in El Niño summers than ENSO-neutral summers (Figure 2), as was the case in 2015 with the registered summer (Jun - Aug) rainfall of 852 mm falling into the "below-normal" category[2].


Figure 2

Figure 2      Distribution of Hong Kong summer rainfall categories under the ENSO-neutral state (left)
and El Niño (right) during 1950-2015.




F C Sham


Notes:

[1] The ENSO-neutral state refers to the situation with neither an El Niño nor a La Niña in place, i.e. no warm (El Niño) or cold (La Niña) sea surface temperature anomalies in the central and eastern equatorial Pacific.

[2] Categorization of rainfall is based on the normal distribution constructed from climatological data in the previous 30 years. The categories are defined in such a way that the highest 30% and lowest 30% of the normal distribution are classified as "above-normal" and "below-normal" respectively, while the middle 40% represents the "normal" range.