From Wednesday until Oct. 21, a “special heatstroke warning alert” will be issued when unprecedentedly dangerous heat levels are forecast for a particular prefecture, according to the Environment Ministry. The special alert, which is one rank above the “heatstroke warning alert” — also to be enacted during the same period — will be issued on a prefectural level if the heat index is thought to surpass 35 degrees Celsius at most observation points in the prefecture the following day. The heat index, known as Wet Bulb Globe Temperature, is calculated using factors such as the temperature and humidity, as well as the surrounding thermal environment, to reflect a more realistic heat level that people experience.