The primary condition for Dokdo’s visibility from Ulleungdo 87.4 kilometers away is visible distance. Rain, snow, fog, dust particles, or other pollutants may block visibility, making the view murky. That is because the vertical movement of air or wind affects the spread patterns of rain drops, cloud drops, or fine solid particles. For this reason, we investigated the meteorological patterns observed in Ulleungdo and determined the weather phenomena that affect the visible distance. In addition, we tried to determine weather conditions under which Dokdo was clearly visible by analyzing common features of the days on which the island was visible.
1. Objective Evidence of Dokdo’s Visibility from Ulleungdo
Dokdo’s visibility from Ulleungdo is a well-established fact by myriad witness accounts of Ulleungdo residents supported by photographs. For the 18-month investigation between July 2008 and December 2009, there was no month in which Dokdo was not visible even for one day. The fact that Dokdo was visible at least three or four times a month on average confirms Dokdo’s visibility from Ulleungdo Another study proved that one can see Dokdo on any point 86 meters or higher on Ulleungdo, based on the Pythagorean theorem and light refraction.who climbed the 984 meters of Seongin Peak, “There is nothing around except the sea waters and the sky.” That is because there were many days in which the upper part of Seongin Peak was covered with clouds. One can thus see Dokdo better from the lower part of the peak where there are no clouds. The Dokdo Observatory, located at a height of 317 meters in Manghyang Peak, is the ideal spot to see the island. As there are many residents living at this height, they take Dokdo’s visibility for granted.
2. Rain and Snow: Why Visibility Worsens in Summer and Winter
1) Eyewitness Accounts by Ulleungdo Residents
How many people in Ulleungdo have indeed seen Dokdo? Of the eyewitness accounts by Ulleungdo residents, a unique one follows. According to accounts by Gwon Gyeong-sun, who lives in a spot where Dokdo is visible right in front of her garden, Choi Hwi-chan, who spent 18 months taking photographs of the island from spots all over Ulleungdo while keeping daily logs, and Lee Deok-jun, who lives in Seokpo Village, in Ulleungdo, it rained or snowed just before Dokdo was visible. This is a question that long nagged the Ulleungdo residents.
〈FIG 15〉Gwon Gyeong-sun (left) is pointing in the direction of Dokdo from the garden of her house.
〈FIG 16〉A picture taken with Choi Hwi-chan (middle, in the photo on the right) who took photographs of Dokdo on the horizon every day through the electricity cable.
2) Rain and Snow Precipitation Depending on Changes in High and Low Pressure Fronts
Given the geographical conditions of Ulleungdo and Dokdo, the islands are subject to pressure fronts moving across the East Sea. For Dokdo to be visible from Ulleungdo 87.4 kilometers away, the visibility distance must be very far, which must be exceptionally clear days.
〈FIG 17〉Four Major High Pressure Fronts Affecting the Korean Peninsula
The eyewitness accounts that it rained or snowed just before Dokdo was visible means that clear weather follows a rainy or snowy spell. According to this reasoning, one must take into account the flow of pressure fronts. On very clear days, it is likely that the weather is under high pressure fronts. As the Korean Peninsula is under the influence of westerlies, the fronts may move east, which means that low pressure fronts would follow high pressure ones and vice versa. Given the pressure front movements in middle latitude regions such as Korea, the residents’ accounts seem more plausible.
Of the high pressure fronts that develop in the Korean Peninsula, the four fronts including the Siberian High pressure front, the North Pacific High pressure front, the Okhotsk High pressure front, and a migratory high pressure front called the Yangtze River Air Mass are the most influential. Korea’s weather patterns are determined by combinations of these fronts.
〈FIG 18〉Dokdo Seen in Spring (March 5, 2009) and Autumn (September 18, 2009)
Spring and autumn are seasons highly subject to migratory high pressure fronts and low pressure fronts. During spring months, westerlies move south in the Pacific Ocean due to the expanding Siberian High pressure front and move back north as the Siberian High pressure front weakens. Then low and high pressure fronts that develop in China would move along with westerlies, affecting the weather patterns in the Korean Peninsula. In autumn, again, westerlies that move north because of the strong North Pacific High pressure front would move back down to the south as the North Pacific High pressure front weakens, thereby making a significant impact on the Korean Peninsula’s weather patterns. At the same time, high and low pressure fronts in the Chinese area of the continent would move east, affecting Korea’s weather conditions. In this case, high and low pressure fronts tend to move together. That is why low pressure fronts follow soon after high pressure fronts, and vice versa, partly supporting the eyewitness accounts that it rains or snows right before Dokdo becomes visible.
〈FIG 19〉This photograph was taken on July 29, 2010, in the front garden of the houses of Gwon Gyeong-sun and Jeong Bong-gweon. One can see that the horizon is not clearly visible, even though it was daytime, due to thick sea fog.
During the summer months, the weather is subject to the North Pacific High and the Okhotsk High pressure front. The Okhotsk High pressure front affects the Korean Peninsula during the months of June and July in the form of the rainy monsoon season. The hot, humid North Pacific High pressure front produces plentiful vapor in the air, creating a situation in which Dokdo is not visible even on clear days. According to an account by an employee at the Ulleungdo Weather Observatory, the surrounding coastal areas of the island were often shrouded with sea fog that resembled sauna steam. At the end of July, when we traveled to Ulleungdo to investigate weather conditions under which Dokdo was visible, we witnessed a steamy horizon due to thick sea fog〈FIG 19〉. Sea fog occurs at sea. It comes about when warm air moves onto the cold sea surface. Sea fog occurs between April and October in Korea. The month in which sea fog occurs most frequently is July. The areas with the highest frequency of sea fog include Gyeonggi Bay, the South Sea’s middle area, and waters near Ulleungdo. For this reason, the best time to see Dokdo in summer months would be right after a shower that would clear vapor from sea fog in the air. This corresponds to the eyewitness accounts that Dokdo was visible immediately after a rain.
〈FIG 20〉Dokdo was visible with the naked eye (January 29, 2009), but in photographic images the island appeared steamy-looking.
In winter months, the weather depends largely on the expansion and weakening of the Siberian High pressure front. As the cold, dry Siberian High pressure front passes through the East Sea, it absorbs vapor in the air, causing snow to fall. Just as in summer months, one is likely to see Dokdo soon after it snows in winter months.
Regardless of the season, vapor in the atmosphere and dust particles in the air fall to the ground due to rain. After the water vapor and dust particles that were blocking visibility through the scattering of light and light absorption are removed, visibility improves markedly allowing Dokdo to be clearly visible. But Dokdo’s visibility before rain may be due to low pressure fronts that arrive after the flow of high pressure fronts.
3) Poor Weather Conditions in Ulleungdo: Rain and Snow Fall Every Few Days
As examined in the section above, there may be several different explanations for why Dokdo was visible before or after it rained or snowed. But we attempted to analyze the phenomenon from different angles as it is difficult to draw a conclusion from a single factor. The following is an inference based upon the precipitation patterns in Ulleungdo to explain the parts not fully accounted for by the flow of pressure fronts. In our high school days, we learned, together with the photographs of udegi (snowwall) and tumakjip (thatched house), that Ulleungdo has even precipitation patterns throughout the year, with high snow precipitation during winter months. But it may seem unfamiliar if one says that it rains and snows frequently in the island. Although one may assume even precipitation throughout the year, it is better to cite weather data gathered by the Korea Meteorological Administration.
〈FIG 21〉Number of days with precipitation of 0.1 millimeter or more across the country (Korea Meteorological Administration, 2001).The darker the color means the higher the number of days of more than 0.1 millimeter of rain. One can see that there were more rainy days in Ulleungdo and the Daegwan Pass than anywhere else.
According to weather analysis data, it is clear there were more cloudy days than clear days in Ulleungdo. As the data are a compilation of weather patterns for 30 years, it is safe to say they are significant statistically. The number of days with precipitation of 0.1 millimeters or more can be seen in the figure below.
As may be seen in the weather map indicating the number of rainy or snowy days with precipitation of over 0.1 millimeter, Ulleungdo had between 140 and 160 such days. This contrasts well with the Seoul Metropolitan area and the inland region (90-110) and with North Gyeongsang-do (80-100)
〈FIG 22〉Number of Days Each Year with Precipitation over 0.1 millimeter from 2001 through 2009 (ranges between 135 and 166 millimeters a year)
The number of days each year with precipitation of more than 0.1 millimeter in Ulleungdo was 135 and 166 millimeters from 2001 through 2009. From this one can say that it rains or snows every few days in Ulleungdo.
In the inland region, North Gyeongsang-do, and South Gyeongsang-do, the greatest amount of precipitation occurs in the early spring, the summer monsoon season, and the early autumn. To better understand the different weather patterns in Ulleungdo, we investigated the number of days of snowfall on the island.
Unlike most regions where precipitation is concentrated in the summer months, Ulleungdo experiences significant precipitation in the form of snow during the winter months.
Snowfall occurs in November, December, January, and February. The number of days is limited to 120 or 201. Of these as many as 50 to 60 days that are snowy, meaning that it snows an average of every other day.
〈FIG 23〉Days Each Year with Snow Precipitation (Korea Meteorological Administration, 2001). This image shows that the darker the color, the more frequently snow precipitation occurs. From this one can see that snows fall frequently in the Daegwan Pass and Ulleungdo.
4) Did It Indeed Rain or Snow before or after Dokdo was Visible?
According to the Dokdo visibility study report, there were 56 days in which Dokdo was visible during the period from July 1, 2008, to December 31, 2009. We wanted to determine whether this was due to the flow of high and low pressure fronts, but we abandoned that idea because 18 months would be too short to analyze the issue in terms of pressure front changes that occur every season. For this reason, we focused rather on meteorological phenomena described in the report while investigating the degree of precision of the claim of precipitation before or after the times when Dokdo was visible.
The 56 days in which the island was visible were divided into five categories: (1) precipitation 1-3 days before visibility; (2) precipitation 1-3 days after visibility; (3) precipitation 1-3 days before and after visibility; (4) no precipitation either before or after visibility; and (5) precipitation on the day of visibility. The results are in 〈Table 11〉 below.
〈Table 11〉 Did Precipitation Occur within Three Days before or after Dokdo’s Visibility from Ulleungdo?
|Precipitation 3 days before visibility|
Precipitation 3 days after visibility
Precipitation 3 days before and after visibility
No precipitation either 3 days before or after visibility
Precipitation on the day of visibility
|Total number of times (days) in which Dokdo was visible||56 times|
Rain or snow fell within three days before or after a visibility episode 39 times, accounting for about 70 percent of the times when Dokdo was visible from Ulleungdo. There were 41 days when precipitation occurred on the day of visibility, which represents 73 percent of all visibility episodes.
Rainfall significantly reduces visibility distance due to the hazy view. Visibility declines not only because of raindrops, but also because of the vapor that develops in the atmosphere. Due to the scattering of light and light absorption, objects in front seem to be blurred. For this reason, it is interesting to hear reports that Dokdo was visible during a rainy day. We investigated the rainy days in which Dokdo was visible in more detail.
Dokdo was visible on rainy days as reported in the visibility study at 10 a.m. on August 28, 2008, and at 3 p.m. on September 28, 2009. The rain stopped at 8 a.m. on August 28, 2008, and at 10 a.m. on September 28, 2009.
We compared the visibility data observed with the naked eye every hour at the Ulleungdo Weather Observatory to determine whether Dokdo’s visibility was due to an increase in visible distance or whether there is any relationship between visible distance and Dokdo’s visibility.
During the hours in which it rained, the visible distance was much shorter than at other times. On August 28, 2008, it rained only slightly (0.5 millimeter), while on September 28, 2009, heavy rainfall was reported (59 millimeter). On the day of light rain, the visible distance was 12 kilometers; and on the day of heavier rain, the visible distance was as short as 1.5 kilometers. This demonstrates how the intensity of rain affects the visible distance. It can be easily understood from everyday experience that in a heavy rain visibility is severely limited but improves during a drizzle.
The visible distance after the rain stopped gradually improved from 8 a.m. at the time the rain stopped to a high of 20 kilometers when Dokdo was visible, as indicated in the first image. As can be seen in the second image, the visible distance doubled to 12 kilometers at 10 a.m. from six kilometers an hour prior. When it rained, visibility was low but improved gradually after the rain stopped. From this, we confirmed that rain and the visible distance are intimately related.
The reason for Dokdo’s visibility on a rainy day was that on both occasions visibility improved rapidly after the rain stopped, which allowed the observers to see Dokdo. Strictly speaking, these two days must be included in the cases in which Dokdo was visible after a rain. This must be included in the cases in which Dokdo was visible after a rain.
〈FIG 24〉Photo of Dokdo taken on a rainy day from Dodong, Ulleungdo August 28, 2008 (above); September 28, 2009 (below)
〈FIG 25〉This is a change of visibility distance for two days during which Dokdo was visible despite rainfall. The arrow mark is the time of photo-taking above.
Although the 56 visible days are statistically significant, the observation period of 18 months is too short to be considered significant. If the number of observation months can be increased in later studies, results from those studies might offer more reliable data for more in-depth analysis. In that case, the reliability of the residents’ reports could be enhanced, as well.
In Ulleungdo where it rains or snows every two or two-and-a-half days, it is natural that Dokdo is not visible on rainy days but is visible on days in between precipitation. The statement by the residents that “It rains three days before or after Dokdo is visible” is in that sense reliable with a 73 percent probability.
3. Fog: The Principal Reason for Low Visibility on the Horizon
〈FIG 26〉Number of Observed Fog Days in Korea (Korea Meteorological Administration, 2001). The average number of fog days in Ulleungdo each year is 45.
A meteorological phenomenon closely related with low visibility is fog. The visible distance changes every hour due to fog on land and at sea. The average number of annual fog days in Ulleungdo is 45 (according to the Ulleungdo Weather Observatory), or about one-eighth of the year. Fog lowers the horizontal visibility distance in Ulleungdo, which again reduces the number of days in which Dokdo is visible with the naked eye.
〈FIG 27〉This table shows the number of thin mist days in Ulleungdo by month.
More responsible for low visibility than fog is “thin mist,” or the light vapor covering the surface of the sea water. Fog is usually defined as a case in which the horizontal visibility distance is below one kilometer, while thin mist appears when the visibility is between one and 10 kilometers due to the vapor covering the sea’s surface. The number of thin mist days observed by the Ulleungdo Weather Observatory is as follows:
Episodes of thin mist occurred about 15 days each month between May and August. In thin mist, the horizontal visibility declines below 10 kilometers, making it difficult to see Dokdo. For this reason, Dokdo is visible only occasionally from late spring to the summer months. Although the horizontal visibility is important for seeing Dokdo from Ulleungdo, other conditions on the sea surface are equally important for Dokdo’s visibility. In photographs in which Dokdo is visible, clear photographs of Dokdo showed a clear horizon. When Dokdo appeared unclear in the photographs, the horizon also looked murky.
〈FIG 28〉A photograph of the Horizon and Sunrise Taken by the Author in August 1982 Facing Jukdo from Wadal-ri, Ulleungdo Dokdo is not visible in the photograph as the horizon appears steamy. If the horizon looked clear, Dokdo would have been visible to the right of Jukdo.
〈FIG 29〉Number of Days per Month When Sea Fog Occurred in Ulleungdo (Observed by Choi Hwi-chan)
Located southeast of Ulleungdo, Dokdo looks like a small triangular speck. For this small triangular point to be visible, the horizon must be clear. When there was fog on the sea, Dokdo was not visible. When it was difficult to tell the difference between the sea and the sky, it is impossible to see the island. which indicates that Dokdo is difficult to see about 19 days each month between April and July. This corresponds to the high frequency of thin mist that occurs between the late spring and summer months, confirming the prediction by academic studies on the occurrence of sea fog in Ulleungdo waters. It is not easy to see Dokdo from Ulleungdo during the summer months because sea fog occurs frequently from late spring through the summer season, which lowers the visibility distance to the side where Dokdo is located.
The sea fog (16) responsible for low visibility to the side where Dokdo is located occurred as many as 19 days each month between late spring (April) and summer (July). This corresponds to the occurrence of thin mist that happens frequently in the months between late spring and summer. This has been confirmed by other studies (17) on the occurrence of fog in Ulleungdo. It is not easy to see Dokdo from Ulleungdo during the summer months because sea fog occurs frequently from late spring through the summer season, which lowers the visibility distance to the side where Dokdo is located.
4. Wind and Air Pressure: Wind that Improves Visibility to Dokdo
〈FIG 30〉 Direction and Speed of Winds on Days in which Dokdo Was Visible in 2009 (left) and Days in which Dokdo Was Visible in September 2009 (right), meters per second
〈Table 12〉 Satellite Images, Weather Charts, and Visibility Distance on Days When Dokdo Photographs were Taken
We carefully examined photographs of Dokdo taken in 2009 by Choi Hwi-chan, a resident in Ulleungdo. Of these, seven photographs, those snapped on the first, fourth, fifth, ninth, eighteenth, twentieth, and twenty-eighth, were taken in September (19.4%). On these days, the principal directions of the wind were north and northeast.
Of the days in which Dokdo was visible in 2009, wind blew from the north or northeast 31 of the 36 days. This means that cold, clean air moved toward Ulleungdo when northern or northeastern winds blew on days in which Dokdo was visible. This corresponds to the phrase “visible on clear days” (86%) in the historical records and makes us admire the keen observation of our ancestors.
For the 56 days in which Dokdo was visible, we analyzed the air pressure distribution pattern based on the surface weather chart. From the analysis, Ulleungdo and Dokdo were subject to high pressure fronts more than 86 percent of the time. We analyzed images from the MTSAT-1R satellite taken at the corresponding times and found that visibility was high with minimal clouds or vapor in the air, which appeared as white spots on the images. That means visibility was high toward Dokdo when high pressure fronts lie in the middle East Sea where the two islands are located.
Even in situations where the visible distance was below 20 kilometers, Dokdo was often visible when high pressure fronts appeared during the day. Even for the remaining 15 percent of the days when there were no high pressure fronts, there were days in which Dokdo was visible. That means one must take into account many other factors when examining the visibility issue.
- [note 144]
- Jeong Tae-man (2008), ibid., pp. 166-200.
- [note 145]
- Ulleung County Journal Publication Committee (2007), “Ulleungdo Inspection Report,” Ulleung County Journal, p. 1379.
- [note 146]
- Korea’s Weather Map, Korea Meteorological Administration, 2001. This data was compiled from 30 years of average weather values (1971 to 2000), and complies with the standards established by the World Meteorological Organization for composing weather maps. This weather map was made using average values of data observed at observation centers and weather observatories across the country.
- [note 147]
- For data on sea fog, refer to the Northeast Asian History Foundation (2008, 2009), Dokdo Visibility Day Study Report.
- [note 148]
- Seo Jang-won, Oh Hwi-jin, An Jung-bae, and Yun Yong-hun (2003), “Studies on the East Sea’s Fog Prediction System,” Korean Society of Oceanography, pp. 121-131.