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:

$Q_{d,t}=Q_{gl,t}\cdot f_{transmission,t}$

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 f_{transmission,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.

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