Chapter 4 ______________________________________________________ Functional Description
VAISALA_______________________________________________________________________ 51
the VV hits. Otherwise, the lower hits are also considered as normal
cloud hits with cloud height (VV + signal range) of /2, that is, VV hits
are assigned to cloud height categories.
The sky condition algorithm has been implemented using five
independent modules. The algorithm control flow is shown in Figure 19
below.
Figure 19 Control Flow of Sky Condition Algorithm
In the control flow, the Initialize module selects ceilometer
measurements from the last 30 minutes, sorts them into time order and
distributes them into specific data structures. After the ceilometer
measurements have been distributed, the module checks whether the
ceilometer has provided enough data for the algorithm. Two ratios are
calculated: ratio of measurements during the last 30 minutes and ratio of
measurements during the last few minutes. If either of these ratios
exceeds a threshold value, the data is marked valid and will be used by
the rest of the algorithm. Both of these ratios require a fixed sampling
period that is automatically detected from the measurements. If there is
no valid ceilometer data, 99 is returned as sky cover value, and if more
than 30 minutes has elapsed, -1 is returned as sky cover value.
The
Filter module converts ceilometer measurements into cloud hits. If
the ceilometer measurement contains multiple cloud bases, the
conversion results in multiple cloud hits. Vertical visibility measurements
are also converted into cloud hits. After ceilometer measurements have
been converted, the amount of clear sky, the amount covered by cloud
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