Heat Module theory

This page described the algorithm prescribed by the DPRA report.

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

Calculate once at the start:

the sky view factor, based on the terrain height, buildings and foliage height.
Calculate the average vegetation fraction and average sky view factor. The average window is based on a wind direction and speed. For more information on this averaging window, see average step.
Calculate per cell 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;
Per cell: does it lies in sun or if sun rays are blocked by a building, foliage or landscape features and therefore is shaded. See Shade calculation;
Per cell: the wind speed at 1.2 m above ground, translated from the 10 m wind speed measured at a weather station.
Per cell: the diffuse radiation, based on the sun radiation and altitude angle. See diffuse radiation formula;
Per cell: 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.