Temperature readings in Tokyo set to drop

The temperature readings in Tokyo this winter are expected to drop because of the relocation of the observation site for Tokyo's weather. On Dec. 2, the Japan Meteorological Agency plans to relocate its observation site for gathering meteorological data 900 meters west of its current site within the agency's premises in the Otemachi district of Chiyoda Ward, Tokyo, to Kokyo Gaien National Garden's Kitanomaru Park, next to the Imperial Palace Grounds.

With the relocation, average temperatures are to be indicated based on relevantly adjusted data for the new site. As the observation environment will change from one surrounded by office buildings to a place inside a park with a lot of trees, the annual average low temperature will drop by as much as 1.4 C, according to agency officials. The number of "winter days" when the lowest temperature drops to below zero also is expected to increase nearly fourfold on annual average.

On an elevated place surrounded with trees in Kitanomaru Park, which is about 10 minutes on foot from the agency's office building, there is a weather observation site called "rojo," where rain and heat gauges and other measuring equipment are installed. This will be the new weather observation site for Tokyo temperatures.

Akiko Yoshioka, 64, a homemaker who often visits the park for a walk, said, "I feel cool here as there are many trees and a moat [of the palace] is nearby."

The current "rojo" observation site is located in the premises of the agency surrounded by office buildings. However, the agency plans to move its office buildings to Minato Ward, Tokyo. For this reason, the agency searched for a new location near the current site to maintain continuity of observations, and chose Kitanomaru Park as a place where the surrounding environment is unlikely to change in the future.

In preparations for the relocation of the observation site, the agency obtained weather data such as temperature and amount of rainfall in Kitanomaru Park for two years beginning in April 2012. After comparing the data at the park site with those of the current site, the agency adjusted the data to what they would have been if observations had been conducted in the park for the past 30 years and calculated new annual average temperatures.

Among new annual average temperatures to be used from Dec. 2, the highest average temperature will remain almost the same. But the lowest average temperature will be 1.4 C lower, at 11.6 C, compared to the current 13.0 C. The annual average temperature also will be 0.9 C lower at 15.4 C, compared to the current 16.3 C.

Regarding the reason of the changes, an official of the agency's observation department said: "Buildings release heat they accumulate in daytime at night, when the temperature drops. So the lowest temperature gauged in the current observation site, which is surrounded by office buildings, does not drop as much as [it does at the new site.]"

An index based on the lowest temperature will also change considerably. The number of "winter days," when the lowest temperature drops below zero, is currently 5.8 days on annual average. However, it will become 20.5 days, a nearly fourfold increase, based on new annual average temperature data. Also, the days of "sweltering nights," when the lowest temperature does not drop below 25 C, will become 11.3 days, less than half of the present 27.8 days on annual average, according to the agency.

The observation site has been relocated before. However, in the past, it was a convention that the same annual average data were used even after relocation. This is the first time that average temperatures will be adjusted to reflect those measured at the new site, according to the agency. An official said, "Please feel at ease, as it does not mean that Tokyo's climate, itself, will change."