Temperatures rose to dangerously high levels in many parts of Japan on Thursday, with the mercury in the city of Shizuoka hitting a record 39.3 degrees Celsius and authorities issuing heatstroke alerts for 16 regions across the country.

The mercury topped the 35-degree mark in cities ranging from Shingu, Wakayama Prefecture, to Kofu, Yamanashi Prefecture, and Shimanto, Kochi Prefecture. The temperature in Tokyo also hit 35 C just after noon — the first time so far this year.

The Meteorological Agency classifies days topping 35 C as “extremely hot days.”

In Shizuoka, western wind going over the mountains caused what's known as the foehn effect, in which wet and cold air climbing up the mountains turns into dry, warm air as it descends, according to Weathernews, a private weather forecaster.

The Environment Ministry and Meteorological Agency issued heatstroke alerts for the prefectures of Ibaraki, Saitama, Chiba and Kanagawa as well as Tokyo’s central area for the first time this year. Alerts were also issued for the Kyushu region and Okinawa Prefecture along with the prefectures of Wakayama and Tokushima.

Heatstroke alerts have been issued for 11 prefectures on Friday, including Chiba, Shizuoka, Aichi and Kyoto.

The ministry and agency issue two-tier heatstroke alerts based on the wet-bulb globe temperature, an index used to measure major environmental heat factors such as air temperature, radiant heat and humidity.

Environment Ministry guidelines stipulate that heavy exercise should be avoided when the WBGT index is between 28 and 31, which carries a "severe warning." When the WBGT index exceeds 31 — classified as "danger" — all forms of exercise should be stopped, according to the guidelines.

As of 1 p.m. on Thursday, Tokyo's WBGT index was 33.5, the second-highest below that of the city of Ebina, Kanagawa Prefecture, which was 34.8.

The Environment Ministry called for people in regions where heatstroke alerts have been issued to stay in cool environments, such as indoor spaces with air conditioning. It also urged people to take frequent breaks and stay hydrated.

Elderly people and younger children are especially vulnerable to heatstroke.

High temperatures have already caused people to get sick, with 2,276 rushed to the hospital in an ambulance in the week through June 30 for suspected heatstroke, according to Fire and Disaster Management Agency data.

Japan is highly likely to suffer another scorching summer this year after logging the hottest summer ever recorded in 2023, the Meteorological Agency has said.

Heat waves are being made stronger and more likely because of climate change, which is primarily caused by the burning of fossil fuels.