Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 143)

131
Figure C 4.3a. Relationship between the shadow disc
and the sensor of a pyranom eter.
Figure C 4.3b. Detail of the sensor projection
on to the norm al plane parallel to the
shadow disc.
C 4.4 Sun at an arbitrary zenith angle
The relationship between a pyranom eter sensor and the shading disc is illustrated in Figure C 4.3a.
The details on the geom etrical situation on and near the sensor is presented in Figure C 4.3b. In this
r'
case the irradiance F at r' on the projected surface m ust be integrated for the surface of the ellipse
which is projected on the plane norm al to the direction of the sun. The solar zenith angle is designated
to be q, then the distance from the centre of the ellipse to its edge l will be expressed as:
Then the absolute am ount of radiative flux for the entire surface will be,
The irradiance for the unit sensor surface on a horizontal surface will becom e
The resuts of the com putation of irradiance for the standard pyrheliom eter and various diffusom eters
are presented in Table C 4.1. From this result it can be concluded that m ost diffusom eters used in
the BSRN show less than 3 W m influence. The best proposal is presented in the second right hand
-2
colum n in the table.