Ceilometer CT25K
User’s Guide CT25K-U059en-2.1
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Overcast (8 oktas) is reported if all the measurements during the last 30
minutes have a hit. Overcast height is assigned according to the bin where
14/15 of the total count value is exceeded.
However, overcast is not reported if there are "weak hits" during the latest 15
minutes period. A "weak hit" is defined as a backscatter signal which is less
than 80% of the reference. The reference value depends on the previous
measurements, i.e. it’s a kind of sliding average value of signal strength.
Vertical visibility (VV) hits are considered as cloud hits if higher than 1500
feet. The cloud height is defined as (VV + signal range)/2. Lower VV hits are
assigned to a VV register. If the percentage of VV hits in the register during
the last 10 minute period exceeds the VLIM parameter value (see Table 4-1),
the sky condition algorithm reports VV. The VV height is the average height
of VV hits. Otherwise the lower hits are also considered as normal cloud hits,
i.e. VV hits are assigned to cloud height categories (bins).
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