Number of hot days

Sector Heat stress and human discomfort
Description Number of days per year with a mean air temperature at 2 m above ground above the 75th percentile during summer months (Apr-Sep). Note that the number of days are calculated over all months of the year, while the percentile has been calculated only over summer months (Apr-Sep).
End User Health authorities, environmental authorities, general public
Calculation method

Temperature 75th percentile is calculated from Harmonie model output during Apr-Sep at the location of an official weather station in the city and then used in the evaluation of each grid cell. The temperature 75th percentile is calculated separately for the historic period and for the present window of the climate scenario (the latter temperature threshold is used also for the future window of the climate scenario). Once the temperature 75th percentile is determined, the evaluation over the grid will performed for the whole year (this since it was observed that for the future climate scenario there are hot days outside the Apr-Sep period).

ID Title Period Statistical processing Unit Threshold Comment
hotdays Hot days > 75th percentile yearly Number of days per year with mean temp > 75th daily percentile. days  

Percentile based on summer temperatures (Apr-Sep)

Provenance Theese indicators are based on output from the Harmonie meteorological modell.
Validation The simulations made by HARMONIE-AROME in Urban SIS has been validated against observations in Urban SIS deliverable 5.1. In section 4.1.3 the simulation for the number of hot days is compared to observations.
Calculation caveats

Spatial representation:
Other caveats: O3, O6
Could be compared to:
Could be used with:


To be consistent with health impact calculations, the threshold that has been chosen to identify a hot day is the 75th percentile of daily mean temperature during summer (Apr-Sep). The term hot days has been chosen instead of the more commonly used heat-wave days since we focus on events that happen many times every year. The commonly used term heat wave is usually defined as a more extreme event. 

Experience user