# 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:

Calculate once at the start, per cell:

- The foliage height in case the foliage height should be based on foliage Functions with a height and a foliage crown factor. See the foliage height calculation model;
- The sky view factor, based on the terrain height, buildings and foliage height; (See Sky view calculation)
- The average vegetation fraction and average sky view factor. The average window is based on a wind direction and speed; (See average step)
- The urban heat island effect value; (See urban heat island formula)

Next, for each timestep, calculate the following, per cell:

- The atmospheric temperature and the Wet-bulb temperature; (See temperature formulas)
- Is the cell shaded by a building, foliage or landscape features; (See shade calculation)
- The wind speed at 1.2 m above ground, translated from the 10 m wind speed measured at a weather station; (See Wind calculation)
- The diffuse radiation, based on the sun radiation and altitude angle; (See diffuse radiation formula)
- The physiological equivalent temperature (PET). If it is night or the cell is in the shade, the shade formula is used, else the sun formula is used.

## Notes

- Each date-time created in the Heat Overlay 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.