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News Navigator: Is the air temperature hotter in Japan than officially announced?

A temperature gauge, which is part of the Automated Meteorological Data Acquisition System, is seen 1.5 meters above the ground on Nov. 4, 2011. (Photo courtesy of the Japan Meteorological Agency)

The Mainichi Shimbun answers some common questions readers may have about the difference between the actual air temperature and that officially announced by the Japan Meteorological Agency (JMA).

Question: A deadly heat wave has hit Japan this summer. Thermometers placed outside some homes showed the temperature at 45 degrees Celsius. Is this the highest temperature in Japan?

Answer: While the highest temperature that the JMA officially announced was 41.1 degrees Celsius measured in Kumagaya, Saitama Prefecture, on July 23, 2018, the JMA heard claims that the real temperature was actually higher. Thermometers at homes sometimes show higher temperatures than those announced by the JMA.

Q: Why is there such a difference between these two temperatures?

A: The JMA's temperature gauges are located in stainless steel air funnels set up at a height of 1.5 meters from well-ventilated lawns. The gauges have heat insulations installed in them and allow outside air to enter inside, which makes it possible to keep a stable temperature. On the other hand, the temperature measured by thermometers at homes, which are often set up outside in summer, rises easily because the devices are vulnerable to direct sunlight, reflected heat and stagnating air.

Q: What is the difference between the temperature of thermometers at homes and the JMA's temperature gauges?

A: It is not surprising if there is a difference of up to 20 degrees between the two devices. JMA Director-General Toshihiko Hashida said at a news conference, "We have set up temperature gauges in the shade, so the air temperature could be 50 or 60 degrees Celsius in a sunny place."

Q: So the difference in air temperature depends on the location?

A: In June 2018, the Automated Meteorological Data Acquisition System in Tatebayashi, Gunma Prefecture, which is well known as a hot city in summer, was relocated from a residential area surrounded by paved roads to a place surrounded by farms and rice fields. After the relocation, the highest temperature at the new observation spot was on average 0.5 degrees lower than that of the former observation area.

Q: So cities are hotter than the countryside, aren't they?

A: It is necessary to warn children, who are smaller than adults and vulnerable to reflected heat from the ground, about the dangers of heatstroke. We should be careful about increases in body temperature when humidity is high or there is little wind.

(Answers by Tomohiro Ikeda, Science & Environment News Department)

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