# Calculation models and formulas (Heat Overlay)

This page describes the algorithm prescribed by the DPRA Heat stress report. For a visual overview of the Heat Stress Module, see Heat Stress Overview.

## Algorithm

The calculation prescription of the DPRA report is implemented as followed:

At the start of the calculation, the following data is pre-calculated for each cell:

- The foliage height calculation model is used to calculate the foliage height (if applicable).
- The sky view calculation model is used to calculate the sky view factor.
- The average calculation model is used to calculate the averaged vegetation fraction and the averaged sky view factor.
- The urban heat island formula is applied to calculate the urban heat island effect value.

Then, for each timeframe, the following data is calculated per cell:

- The temperature formulas are applied to calculate the atmospheric temperature, and the Wet-bulb temperature.
- The shade calculation model is used to determine whether the cell is shaded.
- The wind calculation model is used to determine the wind speed (using weather station data).
- The diffuse radiation formula is applied to calculate the diffuse radiation.
- The physiological equivalent temperature formulas are applied to calculate the final physiological equivalent temperature (PET).

## Notes

- Each date-time created in the Heat Overlay configuration wizard will be its own timestep.
- In the DPRA report step description, the atmospheric temperature is only calculated once. However, the Atmospheric temperature formula used an hour of day input parameter and should therefore be performed each timestep.
- We have not implemented an automatic translation to future KNMI WH-scenarios. However, since users can provide their own weather station data, it can be configured manually.