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  • How super is 'Super Moon' ?

  • Thursday, 10th November 2016

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'[1] 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?

Figure 1


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'[2] 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'[3] 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 [4]. 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.

Table 1


The figure below shows the visual difference between the 'Super Moon' on 14 November 2016 (right) and an ordinary full moon (left).

Figure 2


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)


Notes:

[1] 'Super Moon' is loosely defined as a full moon near the perigee.
[2] 'Angular diameter' is an angular measurement describing how large a sphere or circle appears from a given point of view.
[3] 'Distance between the moon and the earth' is measured from their centre of mass.
[4] 'Average value of full moons' is computed based on all full moons in the year.


Related links:

'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


  • Occurrence of Multiple Tropical Cyclones

  • Wednesday, 21st September 2016

Tropical cyclone development over the western North Pacific and the South China Sea was rather active in August 2016 with a number of tropical cyclones forming during the month. At one time on the morning of 20 August, there were four cyclones (Dianmu, Mindulle, Lionrock and Kompasu) in co-existence over the basin.

According to the Observatory's records since 1960, cases with three tropical cyclones co-existing over the western North Pacific and the South China Sea can be found nearly every year. Cases of four tropical cyclones occurring at the same time were less frequent with only four cases since 2000 as detailed in the table below:

Table 1


The highest number of tropical cyclones in co-existence in this basin is five. It happened twice, in 1960 (Figure 1) and 1985 (Figure 2). On the morning of 23 August 1960, Elaine was over the sea areas east of Taiwan, Carmen to the west of the Korean Peninsula, and Della, Faye and Bess over the western North Pacific. Elaine edged towards the coast of Guangdong after moving across Taiwan, necessitating the hoisting of Standby Signal No. 1 in Hong Kong. On the morning of 1 September 1985, Odessa and Pat were over the Sea of Japan, Ruby east of Japan, Tess east of the Philippines, and Skip near the International Date Line. Tess eventually entered the South China Sea and intensified into a typhoon, passing within 200 km southwest of Hong Kong and necessitating the issuance of the Gale or Storm Signal No. 8.

Figure 1

Figure 1      Tracks of Bess, Carmen, Della, Elaine and Faye in 1960



Figure 2

Figure 2      Tracks of Odessa, Pat, Ruby, Skip and Tess in 1985


Given that tropical cyclones typically require a separation spacing of 10 - 15 degrees to develop, and in terms of longitude the basin of western North Pacific and South China Sea has a span of about 80 degrees from west to east (100oE to 180o), there is probably a reason why the maximum number of co-existing tropical cyclones so far is five. Although six is not impossible, but we will have to wait and see.



C.W. Choy and M.C. Wu


  • The unsettled weather

  • Friday, 9th September 2016

In the weather report issued last weekend, this week's weather was expected to be unsettled. Do you know what "unsettled" means here?

"Unsettled" actually means that the weather can be rather changeable or can vary a lot. It may not be rainy all day long, but under suitable conditions, rain or even thunderstorms can occur. In meteorology, there is a term known as atmospheric instability. One of the ways weather forecasters analyse instability is to study the vertical variation of temperatures and the humidity of the atmosphere, which are commonly represented by the well-known K-index. It can be calculated based on upper-air measurement data, and numerical weather prediction model output from computer can also provide forecasters with the predicted values for the next few days. The K-index consists of three parts: (1) temperature lapse rate, which is the extent of drop in temperature with height; (2) moisture content at lower atmosphere; and (3) the degree of moisture saturation at the middle atmosphere. Generally speaking, K-index is higher for larger temperature lapse rate (i.e. higher temperature near surface and lower temperature aloft), higher moisture content at lower atmosphere, and closer to saturation of moisture at middle atmosphere. A high K-index means that convective activity can easily be triggered, resulting in precipitation or thunderstorms.

The Observatory operates an Automatic Upper-air Sounding System and launches sounding balloons twice a day to obtain upper-air meteorological observation. Take the sounding data in the morning of 5 September this year as an example (Figure 1), the near surface temperature was high and the temperatures aloft dropped rapidly with height (red circles in Figure 1). The warm air near the surface rose easily due to its lower density, resulting in convective activity. Besides, the sounding data showed a small difference between air temperatures and dew point temperatures, which meant that the humidity of the atmosphere was high and there was abundant moisture content for producing precipitation. In general, thunderstorms can be triggered when K-index reaches 33, and the index calculated from this set of sounding data was 37. In fact, there were showers and thunderstorms in Hong Kong on that day, bringing 30 millimetres of rainfall generally over the territory. Rainfall even exceeded 70 millimetres over parts of the territory (Figure 2).

Figure 1

Figure 1      The upper-air sounding profile in the morning of 5 September 2016. Small difference between air temperatures and dew point temperatures indicates higher humidity and near saturation in the atmosphere.



Figure 2

Figure 2      Distribution of total rainfall on 5 September 2016.


Although the atmosphere is unstable, we still need triggering mechanism to initiate precipitation and thunderstorms. This includes, for instance, a trough of low pressure or an upper-air disturbance near Hong Kong, which can enhance the uplift of air. There are other factors like mesoscale or small-scale weather systems and orographic effect, which are also considered and analysed by weather forecasters. From now on, when you learn from the Observatory that unsettled weather is expected in the next few days, remember to stay tuned to our more updated and detailed weather assessment!



Stephen P.W. Lau     L.S. Lee