Delta-T Devices SunScan SS1 User Manual page 36

Preferred light and weather conditions
These also will significantly influence your field operations.
Limitation
Time of day
Solar zenith angle
Incident light level -
absolute
Light level - rate of
change
Full overcast, or full
sun in blue sky
Planning for the sun's position
The SunData software on the PDA contains a useful calculator for solar
zenith angles on any date, to help you plan appropriate times to make
your measurements.
In SunData tap on Utilities to
display the Utilities menu and select
Solar Calc.
The default values are taken from
the Site Settings you are currently
using, but can be changed within
the solar predictor without affecting
any settings elsewhere in the
program.
36  What to Measure and How
Comments
Preferably within 3 hours either side of solar
noon depending on the location and season, to
meet the next two conditions.
Measurements are easier when the sun is
high. Probe and BF5 levelling errors become
larger beyond zenith angles of greater than
60, especially for LAI.
Preferably above about 200 mol.m-2.s-1.
Accuracy degrades below this figure.
With the BF5, avoid only the very fastest
changes between bright sun and cloud.
With no BF5: slow-changing conditions
needed. For LAI, with no BF5, slow change of
direct and diffuse components.
SunScan LAI model copes with both, but full
sun will usually give the best results. Broken
cloud is also satisfactory.
SS1 User Manual v3.3