The nation entered the dog days of summer Tuesday as temperatures climbed above 40 degrees Celsius in some areas, prompting authorities to issue a heatstroke warning and encourage the removal of face masks in certain situations.

By 3:20 p.m. Tuesday, temperatures had reached 40.5 C in the cities of Isesaki and Kiryu in Gunma Prefecture, the hottest reading so far this year, while the town of Hatoyama in Saitama Prefecture had hit 40.2 C. Municipalities in Gunma, Tochigi, Toyama, Saitama, Fukushima, Tokyo and Ibaraki registered temperatures around 38 C or 39 C.

Even in the cities of Sapporo and Asahikawa in Hokkaido, which is generally cooler than the main island of Honshu, the mercury rose above 30 C. At the opposite end of the country, the tropical city of Naha in Okinawa Prefecture recorded 33 C in daytime temperatures.

In the city center of Tokyo, temperatures started soaring early in the morning and had hit 30 C by 7 a.m. Having reached 36 C around noon, the trend did not slow into the afternoon.

The Meteorological Agency reported temperatures were 35 C or higher at 229 locations as of 3 p.m. Tuesday, the highest such number recorded this year.

Meteorologists cited a combination of a strong high-pressure system and hot, dry wind that blows down from a mountain, known as a foehn, as contributing to Tuesday’s sweltering weather.

A special heatstroke alert was issued for six prefectures in the Kanto region and in Yamanashi Prefecture, urging people — especially the elderly — to avoid going outside and to stay inside an air-conditioned room. A lower-level heat advisory was issued across a large swath of the country from Hokkaido to Okinawa.

The risk of heatstroke is especially high this summer as more people wear face masks outside to prevent the spread of the novel coronavirus.

The health ministry is advising people to remove face masks to prevent heatstroke if they are outside and two meters or more apart from others, warning that masks trap heat on the surface of the skin and lead to dehydration.

When wearing masks, the ministry cautions people to avoid exercising outside, use air conditioning and drink a lot of liquid even if they are not feeling thirsty.

There are chances of thunderstorms in Tokyo on Wednesday but similar sizzling weather is expected to last through this week in wider western, central and eastern parts of the nation, including forecasts for 38 C in Nagoya and 37 C in Osaka this weekend, according to the weather agency.

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