To discern weather conditions in which Dokdo can be visible from Ulleungdo, we made use of weather data on the two islands and photographs of Dokdo. We analyzed statistically the weather data on the day Dokdo photographs were taken and found seasonal characteristics of days during which Dokdo could and could not be seen.
1. Weather Observation Spots and Visible Distance Observation method
1) Timeline of the Ulleungdo Weather Observatory and Weather Features
Weather monitoring in Ulleungdo began on August 10, 1938, when the Japanese Government-General of Korea established the Weather Bureau’s Ulleungdo Meteorological Station. In November 1985, a new building was completed at the 589-1 Dodong location. On March 13, 1992, the name was changed to Ulleungdo Weather Observatory.
According to Kim Jin-hyeong, who is a former head of the Ulleungdo Weather Observatory, Ulleungdo is surrounded by high mountains, including Seongin Peak, and thus exhibits different weather patterns in the north and south depending upon wind direction.
〈FIG 4〉 The Observation Field in the Ulleungdo Weather Observatory (photograph taken on July 29, 2010)
It is not rare to see snow in mid-November in below-zero temperatures on the island, and heavy snowstorms are often seen until early spring. Precipitation is generally even across the year, but tends to be greater in summer and winter months. During winter months, snow precipitation falls as high as 293.6 centimeters, the highest of any place in Korea. The Nari Basin exhibits the highest snow precipitation of all. The annual average wind speed is 4.4 meters per second, with an average of 206.2 days having strong winds in excess of 8 meters per second. The number of stormy days throughout the year is 54 days. As strong winds blow an average of 12 to 22 days per month except summer months, it is extremely difficult for the residents to make a living. And this is an obstacle to frequent boat trips for residents and tourists to and from the mainland.
The following are weather patterns observed over the past 30 years at the Ulleungdo Weather Observatory (Table 1). The warmest month is August, with the highest temperature reaching 26.5 degrees Celsius. The coldest month is January, with the lowest temperature falling to 1 degree Celsius below zero. Precipitation is concentrated in the months of June, July, August, and September. January is the month with the highest snow precipitation. The annual precipitation is 1,236 millimeters, with relative humidity in excess of 70 percent. The humidity during the summer months from June to September is generally above 80 percent. The number of foggy days is highest in July with 9.4 days, followed by June with 7.5 days. The number of clear days is highest in September, October, and November, with more than four days.
〈Table 1〉Climate of Ulleungdo (1971 – 2000)
|Daily Average Temperature (Degrees Celsius)||1.3||1.6||5.2||10.8||15.4||18.6||22.3||23.4||19.7||15.1||9.5||4.4||12.3|
|Daily High Temperature (Degrees Celsius)||4.2||4.6||8.8||14.7||19.2||21.9||25.3||26.5||23.1||18.7||12.9||7.6||15.6|
|Daily Low Temperature (Degrees Celsius)||-1||-0.8||2.3||7.5||12||15.8||20||21.2||17.3||12.3||6.8||1.9||9.6|
|Average Humidity (%)||70.7||71.1||71||71||72.4||82.1||86.1||85.3||80.3||72.7||69.8||68.8||75.1|
|Number of Foggy Days||0.1||0.4||1.4||3.6||5.4||7.5||9.4||5.5||1.7||0.8||0.8||0.4||3.1|
|Number of Clear Days||2.3||2.7||5||8.2||6.7||3.8||3||3.9||4.2||6.3||5.1||3.6||4.6|
There are three unmanned automatic weather observation systems in Ulleungdo, in Taeha-ri (Seo-myeon), Dodong-ri (Ulleung-eup), and Cheonbu-ri (Buk-myeon), in addition to a manned observation station〈Table 2〉.
〈Table 2〉 Information on Three Automatic Weather Observation Systems
|Taeha||Ulleungdo Weather Observatory||Cheonbu|
|Location||Taeha-ri, Seo-myeon, Ulleung County, North Gyeongsang-do||Dodong-ri, Ulleung-eup, Ulleung County, North Gyeongsang-do||Cheonbu-ri, Buk-myeon, Ulleung County, North Gyeongsang-do|
|Beginning day of observation||March 30, 1990||March 29, 1999||October 25, 2001|
2) Weather Observation on Dokdo
Weather observation on Dokdo has been conducted in the lighthouse since 1991. Since March 1996, an unmanned automatic observation device has been used. And a new device was installed on October 27, 2009. The Korea Meteorological Administration announces monthly data on temperature, precipitation, wind speed, maximum wind speed for each direction, percentage of observation numbers for each direction, and sequential statistical values for temperature, precipitation, and wind speed (Tables 3, 4, and 5).
〈Table 3〉 Automatic Weather Observation Device Installed in Dokdo
|Location||Dokdo-ri, Ulleung-eup, Ulleung County, North Gyeongsang-do|
|Beginning day of observation||March 2006|
The following are analysis results of temperature data measured in 2009 from three locations in Dokdo and Ulleungdo where an automatic observation device has been installed (FIG 5). As the correlation coefficient (R) between the temperature data of Dokdo and Ulleungdo are 0.95 and 0.97, one can see that their correlations are very high.
〈FIG 5〉Correlation between Monitored Data of Dokdo and Ulleungdo from the Automatic Observation Device (2009)
〈Table 4〉 Meteorological Data on Dokdo Measured by the Automatic Observation Device I (2009)
|Elements Quarterly||January||February||March||April||May||June||July||August||September||October||November||December||Previous Year|
|Precipitation Number Amount (0.1 mm)||Total 00-24h||595||15||45||465||550||2082||895||340||235||545||595||6360|
|Maximum amount||1 day||150||5||40||275||265||570||570||115||150||290||160||570|
|Wind (0.1m/s)||Average wind speed||49||36||47||44||44||42||35||33||25||37||40||46||40|
|Maximum instantaneous wind speed||245||180||229||275||235||210||284||186||179||232||222||265||284|
|Maximum instantaneous wind direction||SW||SSW||WSW||NNW||SW||SSW||SSW||S||S||SW||WSW||SSW||SSW|
|Maximum Wind Speed by wind direction (0.1m/s)||NNE||35||12||50||25||23||19||1||2||12||12||9||50|
|Percentage of observed Numbers by wind direction (0.1%)||Fixed temperature||28||26||11||7||37||47||80||34||69||45||31||24||38|
〈Table 5〉 Meteorological Data on Dokdo Measured by the Automatic Observation Device II (2009)
|階級別日数||Daily maximum temperature (℃)||≦||0.0|
|Daily average temperature (℃)||≦||-5.0|
|Daily minimum temperature (℃)||≦||-15|
|Daily precipitation (mm)||≧||0.0||15||3||2||7||9||18||8||8||10||15||14||109|
|Hourly precipitation (mm)||≧||1.0||8||4||3||12||5||5||3||4||11||55|
|Maximum instantaneous wind speed (m/s)||≧||30.0||1||1|
|旬別平均値||Average temperature (0.1℃)||First 10 days||50||66||72||115||152||165||214||221||221||196||141||95|
|Middle 10 days||35||91||123||160||166||214||239||215||178||91||43|
|Last 10 days||47||58||120||153||206||210||232||218||175||106||51|
|Maximum temperature (0.1℃)||First 10 days||70||94||101||137||176||185||241||241||250||217||167||116|
|Middle 10 days||60||115||148||183||210||240||258||241||194||109||61|
|Last 10 days||73||81||143||181||219||231||251||240||198||128||79|
|Minimum temperature (0.1℃)||First 10 days||31||39||55||105||141||173||197||228||188||159||75||21|
|Middle 10 days||12||71||102||139||194||197||219||204||160||90||27|
|Last 10 days||21||33||41||48||38||30||25||19||27||48||35|
|Wind speed (0.1m/s)||First 10 days||40||36||29||41||48||36||30||25||19||27||48||35|
|Middle 10 days||55||68||37||55||34||61||34||23||49||47||52|
|Last 10 days||52||31||53||29||55||24||40||33||35||25||51|
|Precipitation (0.1mm)||First 10 days||330||5||215||985||770||20||180||405||50|
|Middle 10 days||230||5||175||25||425||125||160||60||120||345|
|Last 10 days||35||10||45||285||310||670||180||15||20||200|
3) How to Observe Visible Distance at the Ulleungdo Weather Observatory
〈FIG 6〉 Target visibility data (provided by Kim Bong-jin, head of the Ulleungdo Weather Observatory, 2010) Although Dokdo lies 87.4 kilometers southeast of Ulleungdo, it is depicted in the drawing as being located nearby without regard to the concentric circle’s distance.
Monitoring professionals in observation centers across the country measure visibility distance by checking whether a distant object is visible from the yard with the meteorological instrument shelter. The visible distance is defined as the maximum distance in which one can visualize an object in the distance. Elements blocking visibility, such as water vapor in the air, snowflakes, dust particles, soot from forest fires, or volcanic ash, are observed every one to three hours by the observatory. That data is then sent to the central Meteorological Administration office in Seoul. One set of data that the Ulleungdo Weather Observatory gathers uniquely is the target visibility figures in the form of a drawing in a concentric circle.
〈FIG 7〉Eight directions of the Ulleungdo Weather Observatory (provided by Kim Bong-jin, head of the Ulleungdo Weather Observatory, 2010)To the east and southeast of the observatory, the view of Dokdo is blocked by Manghyang Peak.
Assuming that the position in which the instrument shelter is located is the central point, distant objects such as mountains, buildings, and islands are measured in kilometers with the naked eye of a person having normal eyesight. When weather changes abruptly, the observer must measure visible distance more precisely. At nighttime, the observer relies on external lights such as street lamps and starlight to measure visible distance. For this reason, observers with abundant experience in weather monitoring in the area are required.
From the target visibility data provided by the Ulleungdo Weather Observatory (FIG 6), Jukdo is located 6 kilometers away from the observatory. From the observatory, one is unable to directly see Dokdo 87.4 kilometers away as it is blocked by a mountain (FIG 7). It was one of the difficulties in conducting surveys on Dokdo weather conditions. It was frustrating for the meteorological professional watching the sky 24 hours a day to work in a place where the view toward Dokdo was blocked (FIG 7). For this reason, we climbed further up to where there is no obstacle to viewing Dokdo so that we could take better photographs of the island.
4) Recently Observed Visibility Distance Features of the Ulleungdo Weather Observatory
〈FIG 8〉 Monthly Visibility Distance Frequency for Ulleungdo
〈FIG 9〉Daily and Monthly Visibility Distance of Ulleungdo and Their Average Value (kilometers)
The following is an analysis of the visibility distance reported by a professional at the Ulleungdo Weather Observatory. Let us determine the visibility distance features observed in 2008 and 2009 (FIG 8). The lowest class of visibility distance less than one kilometer, which is marked in red, appears in June and July most frequently. This is largely due to fog and rain, but the number of days in which visibility is low declines rapidly as the season moves toward autumn. Autumn is the season in which the days of visibility distance in excess of 20 kilometers are highest. This is the by far the best season in which one can see Dokdo from Ulleungdo. One can easily see that autumn is a better season than spring to experience the greater visibility. In the order of better to worse, the seasons are ranked autumn, spring, winter, and summer. The following are the values of visibility distance by season (FIG 9). The annual average visibility distance was 12 kilometers in 2008 and 13 kilometers in 2009. Summer is the season in which the daily average visibility distance shows the highest fluctuations between one and 27 kilometers (FIG 9). During the summer months, when the visibility distance is lowest, the frequency of high and low visibility days on the distribution chart with 10 kilometers as the watershed point was also even. During the autumn season, in contrast, the frequency of low visibility days declined markedly.
〈Table 6〉 Days in which Visibility Distance Exceeded 30 Kilometers I
〈Table 7〉 Days in which Visibility Distance Exceeded 30 Kilometers II
The number of days when the visibility distance exceeded 30 kilometers from 2007 to 2009 was five days. For these five days, the daily change of visibility distance, visibility images of the MTSAT-1R satellite, and ground weather patterns are shown in the following tables (Table 6 and Table 7).
On days in which the visibility distance exceeded 30 kilometers, high pressure fronts in the middle East Sea were observed. Whenever there are clouds or vapor in the atmosphere, satellite photographs show them as white spots. But on these days, there were no such spots and the blue ocean is clearly seen in the photographs.
2. Photographs of Dokdo
As there are many residents living on high ground in Ulleungdo, it is natural for them to see Dokdo from their houses and farms. The fact that Dokdo is visible on clear autumn days is well known. But even on some cloudy, rainy, or snowy days, the island is visible. By year-round continuous observation of the island, data on daily weather patterns, coordinates, and observation height have been collected.
1) Spots in Dokdo from Where Photographs were Taken and the Length of Time
To collect data for conditions under which Dokdo is visible from Ulleungdo, we took photographs from July 1, 2008, to December 31, 2009. The photographs were taken in front of the KBS signal tower in Dodong-ri, Ulleungdo. We kept a daily log of whether Dokdo was visible and took photographs of the island when it was visible.〈FIG 10〉 and 〈FIG 11〉 are part of the daily log in which was recorded a variety of data such as date and time of observation, weather, temperature, wave height, state of the horizon, air pressure, and visible distance. Of the two daily records, the first one said Dokdo was not visible at 9 a.m. on September 27, 2008. The second stated that Dokdo was visible with the naked eye at 9 a.m. on September 28, 2008, with accompanying photographs〈FIG 12〉.
〈FIG 10〉This is the log recording that Dokdo was not visible on September 27, 2008. It described the state of the horizon in detail.
〈FIG 11〉This log, too, noted that Dokdo was visible on September 28, 2008, and included accompanying photographs of the island.
〈FIG 12〉This photograph was taken on September 28, 2008, when Dokdo was visible.
The default observation point was the KBS signal tower in Dodong-ri, Ulleungdo. As suggested in the following table, the observer took additional photographs and made investigations elsewhere in Ulleungdo.
Although photographs were taken every day from the same spot, the observation was made only once a day, with its time of observation inconsistent. Considering that there were days when Dokdo was visible only briefly in the morning or in the late afternoon depending on weather conditions, it is understandable that there would have been many more days in which Dokdo was visible but the camera was unable to confirm it. The number of days Dokdo was visible as listed in the report must thus be taken as the minimum number.
2) Characteristics of Dokdo Photographs: Dokdo Seen as a Triangular Point
The number of days Dokdo was visible with the naked eye was 56 but the number of days photographs were taken was fewer than that. To make repeated observations from the same spot, we made use of a GPS device and digital camera. 〈Table 9〉lists the names of the equipment used in the Dokdo visibility study and their purposes.
〈Table 8〉 Locational Information on the 14 Spots where Observation was Conducted: The KBS signal tower was the main place of observation while other places were used only sporadically.)
|Location||Latitude (degrees and minutes)||Longitude (degrees and minutes)||Altitude (m)|
|KBS Ulleung Signal Tower||37 29.061||130 53.644||227|
|House of Huh Won-gwan||37 29.167||130 53.545||276|
|Tomb of Ahn Pyeong-jeon||37 28.697||130 53.116||294|
|House of Park Yong-su||37 28.781||130 53.225||255|
|House of Lim Jae-gyu||37 28.856||130 53.401||217|
|House of Jeong Bong-gwon||37 28.258||130 52.828||133|
|Seokpo Observatory||37 32.788||130 54.067||291|
|Seokpo Shelter||37 32.116||130 54.429||311|
|House of Lee Deok-jun||37 32.062||130 54.403||296|
|Naesujeon Observatory||37 30.339||130 54.443||441|
|Manghyang Peak||37 28.475||130 54.154||281|
|The octagonal pavilion||37 29.386||130 53.240||708|
|Maljandeung||37 30.419||130 52.457||970|
|Seongin Peak||37 29.879||130 52.027||984|
〈Table 9〉 Cameras and GPS Devices Used in Dokdo Photography
|Sony a350 digital camera, Sigma 18 - 200m lens||Photographing|
|Sony pd-850 HDcamera||Photographing|
|Canon EOS 5 analog camera, Canon 75 - 300m lens||Photographing|
|Garmin GPS 60CS||Measurement of coordinates and altitudes of photo shoot spots|
|Korea’s digital topography by Garmin||Coordinates entered into the digital maps|
〈Table 10〉 above is the photo in which Dokdo appeared relatively better than any other photographs taken on days when the island was visible. As can be seen in the photograph, Dokdo appears as a triangular point about the size of a fingernail.
〈FIG 13〉 is a representation in a bar graph of the days in which Dokdo was visible during the survey period (July 1, 2008 – December 31, 2009).
For the entire length of the 18-month survey period, Dokdo was visible 56 days (20 days in 2008 and 36 days in 2009). In 2008, November was the month in which visible days were highest, while in 2009 that month was September. Due to the unseasonably cold summer in 2009, the number of days in which Dokdo was visible during the months of July, August, and September in that year was higher than usual. During October and November in the same year, higher-than-usual temperatures caused frequent sea fog, which reduced the number of visible days. These points will be discussed further in Chapter 3. Although the number of visible days differs every year depending on weather conditions, autumn was the best season to see Dokdo in both 2008 and 2009.
〈Table 10〉 Days when Dokdo was Visible Relatively Clearly and Photographs Taken on Those Days
〈FIG 13〉The number of days Dokdo was visible is indicated by each month for 2008 (above) and 2009 (below). The months with the greatest number of days the island was visible were November in 2008 and September in 2009. In general, autumn was the best season to see Dokdo.
3) Visibility Distance for Dokdo Photography Time: The Need to Analyze Weather Conditions
〈FIG 14〉 Frequency of visibility distance on days when Dokdo was visible. One can understand from this the visibility distance between 20-22 km was most frequent.
As mentioned above, many meteorological factors affect the visibility of Dokdo. Of these, the most direct factor is visible distance. For this reason, the following is the analysis of visible distance for the 56 days.
Of the 55 days of Dokdo observation except the one day when the observation time was not logged, about 55 percent of the days had visibility distance in excess of 20 kilometers while 95 percent had visibility distance of more than 14 kilometers. Although one cannot say confidently that Dokdo is always visible on days when visibility distance is far enough, the visibility distance of at least 14 kilometers must be secured to see Dokdo.
The fact that the visibility distance must be more than 14 kilometers can be a necessary condition, but can not be a sufficient condition for seeing Dokdo. Future studies need to determine other meteological factors that affect Dokdo’s visibility.
- [note 141]
- Kim Jin-hyeong (2001), “Weather Features of the Ulleungdo Region,” Meteorological News, August, pp. 12-15.
- [note 142]
- In 2007, there was only one day, May 20. In 2008, there were four days, June 16, June 19, August 6, and September 27.
- [note 143]
- Northeast Asian History Foundation (2008), Dokdo Visibility Day Study Report; Northeast Asian History Foundation (2009), Dokdo Visibility Day Study Report.