Annular Solar Eclipse: December 26, 2019

Few solar eclipse tracks manage to cover a larger range of climate types than this post-Christmas annular in 2019. Along its journey from the deserts of the Arabian Peninsula, across Peninsular India, through tropical Indonesia, and on to the moderating influences of the mid-Pacific Ocean (Figure 1),  the track samples climates where cloudiness for the month ranges from as low as 7 percent to as much as 97 percent. Such variety ensures that the avid eclipse traveller will find sites with excellent weather prospects, especially in Arabia and parts of India.

Figure 1: Track of the Annular Eclipse on December 26, 2019

The eclipse begins at sunrise over the Arabian Peninsula beneath a weak ridge of high pressure that stretches southward from Iran to Somalia (Figure 2). The track lies at the southern periphery of the Siberian High and north of the Intertropical Convergence Zone (ITCZ), a region characterized by the flow of the Northeast Trade Winds. For the most part, the northeast flow is very light, so that it is locally influenced by terrain, land-ocean boundaries, and weak high- and low-pressure ridges and troughs.

Figure 2: Average December sea-level pressure along the eclipse path. ITCZ denotes the average position of the Intertropical Convergence Zone.

As the track leaves the influence of the Siberian anticyclone and approaches the ITCZ, the dry, desert climatology of the Arabian Peninsula gives way to an increasingly humid tropical environment. The ITCZ is the Earth’s “weather equator,” where Northern Hemisphere northeastlies collide with Southern Hemisphere southeasterlies, forming a zone of very heavy cloudiness and frequent precipitation (Figure 3) — in fact, the wettest area on the globe at this time of year. After reaching its most southerly point near Singapore, the shadow path begins a slow turn to the northeast, pulling away from the ITCZ and returning to the moderately drier weather of the central Pacific.

Figure 3: December precipitation derived by multiple earth and satellite based sensors as part of the Global Precipitation Climatology Project. The blue-toned maximum over Sumatra and Borneo has the highest precipitation rate on the globe at this time of year. Data are derived from 1979 – 2004 measurements. Source: GPCP.

The Arabian Peninsula

The eclipse begins at sunrise in Saudi Arabia, about half way between Damman and Riyadh and astride the highway that connects the two. Cloud levels are moderately high in this region, brought on by occasional weak disturbances from Africa and the Mediterranean that reach the area in winter. Because of the low altitude of the Sun, clouds will be particularly annoying if present on eclipse day. However Figure 4 and the graph in Figure 5 shows that the cloud climatology improves rapidly along the southeastward-trending eclipse track and just a thousand kilometres later, as it crosses Highway 29 in Oman, cloudiness drops to the lowest level along the track — just seven percent according to measurements from the Aqua and Terra satellites (Figure 5).

Figure 4: Average December cloud amount along the eclipse track derived from 15 years of satellite observation. Source: NOAA.

In Table 1, the observed levels of cloudiness, though very good, are not as optimistic as those shown in the graph of centreline cloudiness. This difference seems to be due to the influence of the Arabian Sea, which being cloudier than the land, give coastal cities a cloudier climate than the inland deserts. We can see some evidence of the influence of the coast in the graph of centreline cloudiness near Doha, where the shadow path briefly crosses onto water and the graph becomes more variable. 

Graph of satellite-derived cloudiness
Figure 5: Graph of satellite-based cloud amount along the central axis of the eclipse based on observations from 2002 to 2016. The red line shows cloudiness derived from 10:30 am satellite passage; the blue line is extracted from 1:30 pm passes. Locations of places along (but not necessarily on) the central line are shown on the graphs. Source: NASA.

 

Table 1: Surface observations of weather elements at selected locations over the Arabian Peninsula along the eclipse track . Cloud-cover observations are taken at the time of the eclipse. The average cloud amount is calculated from the individual observations of cloud frequency categories. “X” refers to obscured skies with no sky visible; “-X” to partly obscured skies, Tmax and Tmin are average daily highs and lows; Rtmax and Rtmin are record highs and lows for the month. “POPS” is the “percent of possible sunshine” as determined from sunshine recorders. The cloud category observations were taken at or near eclipse time.

 

Minimum cloudiness occurs in the stony desert of Oman’s interior where the general lack of vegetation gives testimony to the sunny and waterless character of the climate. Happily, there are tourist refuges in the midst of the desert such as the intriguing Qasr Al Sarab Desert Resort in Abu Dhabi that can provide the avid eclipse seeker with an expensive but comfortable viewing site.

December is winter on the Peninsula and winter brings both colder weather and occasional storms. Along much of the track, those storms take the form of the Shamal, a northwesterly wind that typically follows the passage of a dry cold front and is frequently the cause of significant dust storms. The winds are directed down the Persian Gulf by the higher terrain in both Iran and Saudi Arabia or caused by northeasterly winds that blow across the Persian Gulf from Iran.

Fortunately, the winter dust storms are uncommon except along the coast of the peninsula where it may occur 2 – 3 times per month and last 24-36 hours. Longer lasting 3-to-5 day shamals are even less frequent, coming once or twice in the winter season. The region along the UAE-Oman border is most often affected in the winter season, but storms can occur across all of the eclipse track in the Arabian Peninsula. Some impact of the shamal can be judged from the “X” column in Table 1, which shows the frequency of sky-obscuring visibilities. Dust may be carried many kilometres into the air by a Shamal and even if surface visibilities remain high, the hazy atmosphere from suspended particulates will detract from the eclipse experience.

India and Sri Lanka

Cloud amounts rise very quickly as the shadow track leaves the deserts of the Emirates and Oman and crosses over the Arabian Sea. The track is heading southeastward, deeper into the tropical and equatorial moisture, but on reaching the west coast of India, finds a small low-cloud refuge that gives the eclipse seeker another fine location to set up for observation (Figure 5).

At the time of the eclipse, the Indian sub-continent is embedded in the flow of the Northeast Monsoon. For most of north and central India, this is a time of little or no rain, low humidity, and cloud-free skies, courtesy of a flow of cold dry air from Mongolia and Central Asia that must descend from the Himalaya Mountains. In the southern extremities (Peninsular India), the northeast winds must first pass over the warm waters of the Bay of Bengal, where they are filled with moisture. The moisture is released as rain when the flow reaches the eastern coast, and so the southern states along the eclipse track have a relatively wet climate compared to the rest of the country and for some regions, it is the wettest time of year.

Fortunately for eclipse watchers, India’s west coast is largely protected from the moist Northeast Monsoon by the Western Ghats, a mountain range that stretches the whole of the length of the Arabian Sea coast, about 100 km inland. Northeast winds crossing the Ghats are compelled to drop much of their moisture on the inland side, leaving the coastal plain where the eclipse track comes ashore with a drier and sunnier climate. The dramatic impact on cloud cover is best seen in the graph of Figure 5, where cloud amounts average less than 30 percent along the Arabian Sea and 75 percent along the Malabar Coast against Sri Lanka. The same pattern is revealed by observations from weather stations shown in Table 2.

Table 2: Surface observations of weather elements at selected locations over India and Sri Lanka along the eclipse track . Cloud-cover observations are taken at the time of the eclipse. The average cloud amount is calculated from the individual observations of cloud frequency categories. See Figure 1 for an explanation of the columns.

 

Cloud statistics are reflected in the graph of December precipitation shown in Figure 6, where stations along the Arabian Sea coast have about half the monthly precipitation of those east of the Ghats.  Jaffna, in Sri Lanka, exhibits the best of the Northeast flow rainy season with an average rainfall of over 295 mm in December and is not shown on Figure 6.

In spite of the favourable weather prospects along the Arabian Sea coast, it would be wise not to be too complacent about the weather on eclipse day. While sunshine is abundant, Peninsular India is wetter and cloudier than the rest of the country in December. Fortunately the cloud is largely driven by daytime convection, which will be subdued at the morning passage of the eclipse.

Rainfall graph across India and Sri Lanka
Figure 6: Graph of December rainfall amounts along the eclipse track across India and Sri Lanka

Tropical Cyclones

This eclipse comes at a time of year when tropical storms and depressions (a.k.a. hurricanes for North Americans) may form over both the Bay of Bengal and the Arabian Sea. They are relatively uncommon, especially for an eclipse date nearly at the end of December, as the most likely months for storms in the Bay of Bengal are October and November, and in the Arabian Sea, November.

In the Bay of Bengal, a total of 11 storms formed in December in the 25 years between 1974 and 1999 and of these, 7 crossed the east coast of India. Since this is a very long coastline, the number that affected the eclipse track was probably small, especially as Sri Lanka was hit by only 8 December storms in the 130 years between 1889 and 2011. However, 5 of these fell in the last two weeks of December and would have directly impacted eclipse viewing.

Indonesia, Singapore, and the Philippines

The increasing southeasterly track of the Moon’s shadow takes it deep into equatorial moisture and the extensive cloudiness that marks the personality of the ITCZ.  The centreline trace of monthly cloudiness in Figure 5 shows values of over 75% in Sri Lanka, a value that increases a further 10% by the peninsula of North Sumatera (Sumatra).  At Singapore, where totality occurs at noon, satellite measurements of cloud cover reach 95%, a value that seems far too high in view of the approximately 33 percent of possible sunshine at most stations shown in Table 3. However, the surface-based cloud observations are in close agreement, with no observations at all of clear skies and a 93-100% frequency of broken and overcast skies at most of the stations. This is a very cloudy region of the globe, especially in the afternoon.

Table 3: Surface observations of weather elements at selected locations over SE Asia along the eclipse track . Cloud-cover observations are taken at the time of the eclipse. The average cloud amount is calculated from the individual observations of cloud frequency categories. See Figure 1 for an explanation of the columns.

 

If we examine the graph of satellite-based cloudiness closely, there is evidence that the proximity to water brings some relief from the cloudiness, something that we would expect from theoretical grounds. At several times – about 5 – the graph dips downward and these dips can be matched with land-ocean boundaries. One is west of Singapore, one west of Singkawang City, and another near Pulau Miangas. For the most part, it’s only a 10% drop in cloud cover, but in such a cloudy environment, suggests a direction and place to go to watch the eclipse.

As the track draws away from its most southerly point and heads toward the northeast, it gradually leaves the equatorial climate and the ITCZ. Cloudiness drops slowly and steadily from Kalimantan (Borneo) onward so that by the time the shadow says goodbye to the southern tip of the Philippines, the average cloudiness has dropped below 80 percent.

Across the Pacific Islands

Beyond the Philippines, only the U.S. islands of Guam and Rota Island fall under the spell of the eclipse, though some of the islands of Ulithi in the Caroline Islands lie less than 50 km south of the south limit.

Guam lies within the influence of the Pacific’s sub-tropical high and is embedded in a steady northeast trade-wind flow through the year. December is one of the windier months, but it is also the start of the dry season, Fanumnangan, when rainfall begins a modest decline, heading for a minimum in March. Daily weather is often interrupted by passing showers, as the heating of the island and the disturbance of the flow of wind promotes the formation of clouds. The flat low-lying northern half of Guam is most exposed to the wind.

Guam has an average monthly cloudiness of around 65% in the satellite images and 41% according to surface observations though it is variable depending on the lay of the terrain. More reliably, sunshine measurements indicate that 40% of the daytime hours are sunny – a small number compared to those over Arabia, but more encouraging than those along the equator.

Table 4: Surface observations of weather elements at selected locations over the Western Pacific along the eclipse track . Cloud-cover observations are taken at the time of the eclipse. The average cloud amount is calculated from the individual observations of cloud frequency categories. See Figure 1 for an explanation of the columns.

 

A close look at the satellite observations show that the central and  northeast parts of Guam have slightly lower cloud amounts than the southwest, with the largest amounts on the west-facing coast at Facpi Point. Prevailing winds are from the east and northeast, so the cloud maximum is likely caused by the influence of the higher terrain over the southern part of the island. In any event, the difference is barely 5%.

Typhoons occur year round in the Western Pacific but are most frequent in October and November and decline sharply in December. Based on rather dated statistics, only 5 typhoons have passed within 180 km of Guam in the last 10 days of December between 1945 and 1990 – a rate of once every 9 years (NOCC/JTWC Tech Note 91-2).  And while the risk is slight, the effect can be monumental: Super-typhoon Pongsona, with sustained winds of 126 km/h and gusts to 257 km/h, struck Guam and Rota on December 8, 2002. This storm dropped over 580 mm of rain across Guam’s midriff, left the entire island without power, and destroyed 1300 homes.

Summary

Favourable weather can be found at the morning and evening segments of this eclipse track where the path lies farthest from the Intertropical Convergence Zone. The deserts of Oman have the lowest cloud frequency of only 7 percent, probably the lowest value these studies have reported in the past 40 years. It comes with a modest risk of dusty skies and movement along the path could be difficult, as most roads run east and west. The Indian coast along the Arabian Sea also offer excellent cloud statistics, with a greater opportunity to relocate if weather is unfavourable.

May 2017

 

 

 

 

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Climatology and weather for celestial events