Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 137)

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m easurem ent, nam ely the atm ospheric conditions (scattering) and the solar elevation. For a given
pyrheliom eter and given circum stances several such diffusom eters could be com posed that would
result in zero DE. For a given pyrheliom eter + diffusom eter system , DE varies with varying circum stances.
To obtain sm all DE values in each condition, the two penum bra functions should be as close to each
other as possible. The construction of a diffusom eter that (together with the pyrheliom eter) gives yearly
zero-bias global radiation, requires that the yearly com bined probability distribution of:
1. solar elevation,
2. turbidity (scattering),
3. diffuse radiation,
4. sunshine
be taken into account when m inim izing the standard deviation of DE around its zero m ean.
Examples
The above considerations will be applied for:
1. a very wide range of sky functions,
2. 20, 45 and 60 degrees of solar elevation,
3. geom etrically different pyrheliom eters,
4. pyranom eters used in radiation stations
to find the diam eter of shading disk and the length of arm that diffusom eters require for optim al
perform ance.
C 3.2.1 Sky functions
The sky functions (only cloudless atm osphere is considered) depend on:
1. the type and am ount of atm ospheric aerosol,
2. the hum idity (size distribution of aerosol and appearance of haze particles),
3. the solar elevation angle (optical path length).
For the least scattered radiation, a m ountain aerosol m odel was used with an optical depth of 0.05
The m iddle conditions are represented by continental aerosol with an optical depth of 0.23. Also for
these conditions a m easured sky function is used. It is a m ean of circum solar m easurem ents m ade
in Boardm an (USA) at 45 degrees solar elevation. This function is quite close to the one calculated
for rural aerosol plus haze particles with an optical depth of 0.02. The radiation flux with the greatest
scattering is represented by a sum m ertim e desert aerosol of 1.0 optical depth. For desert conditions
there have been reports of optical depth as high as 4, but even in the case of 1 optical depth, direct
radiation hardly could be m easured. Table C 3.1 provides the direct beam irradiance for each of the
m odel and solar elevation com binations.
The calculation of the sky functions has been m ade by M. Putsay.
Solar elevation
(deg)
Mountain
aerosol
Continental
aerosol
Summer
desert aerosol
Measured
60 1041 863 343
45 983 785 253 770
20 727 470 45
Table C 3.1. Direct radiation (W m ) for the 10 sky functions. -2