Very high daytime temperatures with increased health impacts for vulnerable people at risk of hospital admission or death. Increased transport disruption – e.g. track buckling on railways, road melt. Overhead power lines become less efficient. One Hot Summer Day is one day in which the threshold is passed in a year.This data-set contains 3 fields for each fixed period (1981-2000, 2001-2020) and Global Warming Level (1.5°C, 2°C, 2.5°C, 3°C, 3.5°C, 4°C) combination: the median, 2nd lowest and 2nd highest among the 12 ensemble members. The fields are named accordingly; e.g. the 2nd lowest at 2.5°C is HSD_25_lowerTo understand the data, refer to the LACS Scientific Detail.To understand how to explore the data, see the User Guides available on the Climate Data Portal.