Diffuse radiation formula (Heat Overlay)

The diffuse radiation is an input parameter for the PET-formula.

The diffuse radation Q_d at time t is calculated using the following formula:

$Q_{d, t} = Q_{g l, t} \cdot f_{t r a n s m i s s i o n, t}$

where,

$Q_{g l, t}$ is the amount of sun radiation at time t, measured at a weather station.
$f_{t r a n s m i s s i o n, t}$ is the atmospheric transmission factor at time t.

Atmospheric transmission factor

The atmospheric transmission factor f_{transmission,t} is calculated as followed:

where $τ_{a}$ is the atmospheric transmissivity. This is calculated as:

$τ_{a} = \frac{Q_{g l, t}}{1367 \cdot \sin (ϕ_{t})}$

where,

$Q_{g l, t}$ is the amount of sun radiation at time t, measured at a weather station.
$ϕ_{t}$ is the sun altitude angle in ° at time t.
$1367$ is sourced from the DPRA Heat stress report.