How many days in a year can we see Mt.
Fuji and the Tokyo Tower from the Tokyo suburban area?
Photgraphs show Mt. Fuji (left) and the Tokyo Tower (right) veiwing from
Seikei High (2000/1/1)
Introduction
Mt. Fuji and the Tokyo Tower have been observed by human eyes from the
Seikei Meteorological Observatory, Tokyo, every day for the past 30 years.
The data obtained from such unique observation bears a significant relationship
with the air pollution around the Tokyo Megalopolis.
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Method
In 1962, a new schoolhouse of four stories was constructed. Mr. T. Kato,
the head of the observatory at that time, found that Mt. Fuji and the Tokyo
Tower, which are the tallest mountain and the tallest tower in Japan, respectively,
often could be seen from the roof top of the schoolhouse, particularly
in winter. Furthermore, he began to record whether Mt. Fuji and the Tokyo
Tower could be seen from the roof top on 1 January 1963. Since this time,
the staff members of the observatory have been recording only whether each
object could be seen every day at 9:00. The data of such a simple observation,
however, were clearly related to the air dust concentration around the
Metropolis of Tokyo. The observatory is located near the border between
the Tokyo Wards area and the Tokyo Cities area (Fig.1). The Tokyo Wards
area was already urbanized in the 1960s, whereas the Tokyo Cities area
only began to become urbanized in the 1960s. The Tokyo Tower is located
about 17 km southeast of the observatory, and can be seen it over the Tokyo
Wards area. In contrast, Mt. Fuji, located about 83 km southwest-by-west
of the observatory, can be seen over the Tokyo Cities area and the adjacent
mountainous area.
@@@Fig. 1. Locations of the Seikei Meteorological Observatory, Mt. Fuji
and the TokyoTower.
The objects are observed every day at 9:00 a.m. with the naked eye, and
the data relating to whether each object could be seen has been published
in the annual report of the observatory. The subjective visual clearness
of each object, such as "clear" or "vague", is also
recorded in field notes, but unfortunately is not published. The observation
has been maintained for more than 30 years without exception, and the total
number of days when observation has taken place exceeds 11,000 days. Here,
we call a day when the object could be seen as a "visible day".
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Visible days
Figure 2 shows the annual change of the number of visible days from 1963
to 1993. Mt. Fuji could be seen 43 days/yr on average from 1963 to 1972,
and the minimum number was recorded in 1965 (22 days/yr). After 1973, however,
it could be seen about 70 days/yr on average. The Tokyo Tower could be
seen 69 days/yr on average from 1963 to 1972, and the minimum during this
time occurred in 1969 (29 days/yr). From 1967 to 1972, the number of visible
days was fewer than that of Mt. Fuji, despite Mt. Fuji being located about
five times farther than the Tokyo Tower from the observatory. However,
this number tended to increase from about 1972, and its averages were 100
days/yr and 124 days/yr from 1973 to 1982 and from 1983 to 1992, respectively.
Further, the maximum of 159 days/yr was obtained in 1993.
Fig. 2. Time variation of the annual visible days of Mt. Fuji and the Tokyo
Tower.
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Discussion
Meanwhile, air dust concentration (ADC) was measured by the Government
of Tokyo from 1970 to 1984 in the Tokyo Wards area, and from 1972 to 1984
in the Tokyo Cities area (Komeiji et al, 1990). The ADC in the Tokyo Wards
area was 0.130 mg/l in 1970, and this decreased quickly to 0.073 kg/l until
1973. Further, it decreased gradually to 0.054 mg/l until 1984. The ADC
in the Tokyo Cities area was 0.074 mg/l in 1972, and this also decreased
gradually to 0.055 mg/l until 1984. This indicates that the air around
Tokyo became clearer during the 1970s, particularly in the Tokyo Wards
area. The visual clarity of air relates not only to ADC but also to other
meteorological factors, such as air humidity and/or weather conditions.
However, a significant negative correlation was found between ADC and the
number of visible days. The regression coefficient between the ADC in the
Tokyo Wards area and the annual visible days of the Tokyo Tower was -0.94,
and that between the ADC in the Tokyo Cities area and the annual visible
days of Mt. Fuji was -0.82. These facts indicate that the improvement of
air quality around Tokyo is a cause of the increase of the number of visible
days. Komeiji et al. (1990) in his report regarding the history of air
pollution around Tokyo states the following. The air pollution of Tokyo
in the 1960s was caused by coal combustion and insufficient emission control.
After that, major fuel consumption was switched from coal to oil, and dust
collectors were installed in the 1970s. This change reduced particulate
emissions into the air around Tokyo. The increase of road surfacing and
housing has also reduced the generation of soil dust. The observation carried
out by the Seikei Observatory is very simple and can be done by anyone.
However, it has been very hard to continue such a simple observation every
day for more than several decades, particularly considering that it is
a private observatory. Such simple data might also cast light on environmental
changes if observation were to be continued for an indefinite period.
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References:
Komeiji, T., Aoki, K. and Koyama, I. (1990): Atmospheric Environment, 24A,
1099-2103.
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