Temperatures in the Kanto region, eastern Japan, on Monday were the highest of the year so far, with the mercury topping 36 degrees Celsius in a number of areas.
The Meteorological Agency says a Foehn phenomenon, in which an air mass descends from mountains and is dried and heated by compression, added to sunshine on the Pacific side of eastern Japan. The weather pattern caused temperatures to soar in the Kanto Koshin region.
Daytime highs hit a record 36.8 degrees Celsius in Sano City, Tochigi Prefecture, the highest in the country so far this year, 36.4 degrees in the Ushiku district in Ichihara City, Chiba Prefecture, 36.1 degrees in Yorii Town, Saitama Prefecture, and 36 degrees in Kiryu City, Gunma Prefecture, nearly 10 degrees higher than an average year.
Shizuoka City marked 35.5 degrees, Otsuki City in Yamanashi Prefecture had 33.8 degrees, Central Tokyo, Yokohama City, and Matsumoto City had 33.4 degrees, and Mito City reached 33.1 degrees, the hottest temperatures for each so far this year.
In Tokyo, the Fire Department says 20 people were rushed to hospital due to apparent heatstroke as of 3 p.m. on Monday.
Of them, five have shown moderate symptoms, while the remaining 15 have shown light symptoms.
Officials are calling on residents to take liquids before they feel thirsty, to stay indoors and use air conditioning and electric fans accordingly.
Hot weather is expected to prevail on Tuesday. Daytime highs are expected to be 34 degrees in Maebashi City and Kumagaya City.
Sendai City, Utsunomiya City, and Shizuoka City can expect highs of 33 degrees, while Mito City, Toyama City and Naha City will have 32 degrees.
The heat is significantly increasing risks of heatstroke across Japan, and has prompted authorities to issue a heatstroke alert for the Amami region in Kagoshima Prefecture, Okinawa's main island and the Yaeyama region.