Diffuse radiation formula (Heat Overlay)
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The diffuse radiation is an input parameter for the PET-formula.
The diffuse radation Qd at time t is calculated using the following formula:
$ Q_{d,t}=Q_{gl,t}\cdot f_{transmission,t} $
where,
- $ Q_{gl,t} $ is the amount of sun radiation at time t, measured at a weather station.
- $ f_{transmission,t} $ is the atmospheric transmission factor at time t.
Atmospheric transmission factor
The atmospheric transmission factor ftransmission,t is calculated as followed:
| ftransmission,t | Condition |
|---|---|
| $ 1 $ | $ \tau _{a}<0.3 $ |
| $ 1.6-2\cdot \tau _{a} $ | $ 0.3<\tau _{a}<0.7 $ |
| $ 0.2 $ | $ \tau _{a}>=0.7 $ |
where $ \tau _{a} $ is the atmospheric transmissivity. This is calculated as:
$ \tau _{a}={\frac {Q_{gl,t}}{1367\cdot \sin(\phi _{t})}} $
where,
- $ Q_{gl,t} $ is the amount of sun radiation at time t, measured at a weather station.
- $ \phi _{t} $ is the sun altitude angle in ° at time t.
- $ 1367 $ is sourced from the DPRA Heat stress report.
