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:

Qd,t = Qgl,t * ftransmission,t
where,

  • Qgl,t is the amount of sun radiation at time t, measured at a weather station.
  • ftransmission,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 τa <= 0.3
1.6-2* τa 0.3 < τa < 0.7
0.2 τa >=0.7

where τa is the atmospheric transmissivity. This is calculated as: τa = Qgl,t / (1367 * sin (φt))
where,

  • Qgl,t
  • φt is the sun altitude angle in ° at time t.