Vaisala AWS330 - Manual (Page 284)

User's Guide ______________________________________________________________________
282 _________________________________________________________________ M211296EN-B
Sunshine Duration
Sunshine duration calculation reports whether the sun is shining or not.
The sun is considered to be shining, if global radiation (G) is more than
120 W/m2. Also, the ratio G/G0, where G0 is the calculated
extraterrestrial global radiation (on top of the atmosphere), must be over
0.4. Calculation of G0 requires stations position (longitude and latitude),
current (= local) time, and day number (0-365).
The global radiation G is measured with a pyranometer (CM6B is
recommended), using an analog measuring channel. Time and day
number are composed using the QML logger internal time functions. Of
course, user must set local time and time zone correctly to ensure proper
operation.
Before G0 can be calculated, the zenith angle of the sun, at station
location, must be less than 85 degrees. The G/G0 limit, zenith angle
limit, and radiation limit are configurable.
The sunshine duration component returns Boolean-type data '1' if the sun
is shining and '0' if it is not. This data is supposed to be logged by the
QML logger or external system to retrieve a history of sunshine duration.
Calculation of Extraterrestrial Global Radiation G0
) cos(
)
365
4
sin 000077 . 0
365
4
cos 000719 . 0
365
2
cos 034221 . 0
000110 . 1 ( 1367
0
e
DN
DN
DN
G
⋅
⋅
⋅ +
⋅
⋅ +
⋅
⋅ +
⋅ =
π
π
π
where
DN
= Day number (1 ... 366)
e = Zenith angle (-90.0 ... 90.0 degrees)
G
0
= Extraterrestrial global radiation (0 ... 1422 W/m
2
)