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 * 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	τa <= 0.3
1.6-2* τa	0.3 < τa < 0.7
0.2	τa >=0.7

where τ_a is the atmospheric transmissivity. This is calculated as:
τ_a = Q_gl,t / (1367 * sin (φ_t))
where,

• Q_gl,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.