Climate change is increasing the likelihood of health impacts from hot weather. Heat-Health Alerts (HHAs) are issued in England if high temperatures are expected to pose a health risk to some or all of the population. Current public health advice from the UK Health Security Agency advises the public to keep out of the sun between 11:00 and 15:00. This advice aligns with known patterns for Ultraviolet (UV) radiation exposure, but evidence suggests this may not be the hottest time of day. There is a need to update evidence regarding the daily timings for exposures to maximum temperature and make comparisons to UV radiation to inform public health guidance.

Ultraviolet Index (UVI) data from four observation sites across England were matched to temperature data from four weather measurement stations. Data from 2020 to 2024 during HHA seasons was examined. The mean times (British Summer Time, BST) at which maximum daily temperatures and UVIs occurred during the HHA season were calculated, and differences examined through Lowess plots with locally weighted regressions.

19 HHAs were issued in the study period. Daily maximum temperatures occurred on average at 13:56 BST [95% Confidence Interval (95%CI) 13:44; 14:09], while maximum UVI was observed on average at 13:11 BST [95%CI 13:07; 13:16].

Data from four geographically dispersed sites in England over 5 years suggests that daily maximum temperature occurs 45 min later in the day on average than maximum UVI. Results can be used in conjunction with behavioural and communication findings to inform public health guidance.