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Ann. Geophys., 21, 649-654, 2003
www.ann-geophys.net/21/649/2003/
© European Geosciences Union 2003
Using routine meteorological data to derive sky conditions
D. Pagès1, J. Calbó1,2, and J. A. González1
1Department of Physics, University of Girona, Girona, Spain
2Institute of the Environment, University of Girona, Girona, Spain
Abstract. Sky condition is a matter
of interest for public and weather predictors as part of weather analyses. In
this study, we apply a method that uses total solar radiation and other
meteorological data recorded by an automatic station for deriving an estimation
of the sky condition. The impetus of this work is the intention of the Catalan
Meteorological Service (SMC) to provide the public with real-time information
about the sky condition. The methodology for deriving sky conditions from
meteorological records is based on a supervised classification technique called
maximum likelihood method. In this technique we first need to define features
which are derived from measured variables. Second, we must decide which sky
conditions are intended to be distinguished. Some analyses have led us to use
four sky conditions: (a) cloudless or almost cloudless sky, (b) scattered
clouds, (c) mostly cloudy – high clouds, (d) overcast – low clouds. An
additional case, which may be treated separately, corresponds to precipitation
(rain or snow). The main features for estimating sky conditions are, as
expected, solar radiation and its temporal variability. The accuracy of this
method of guessing sky conditions compared with human observations is around
70% when applied to four sites in Catalonia (NE Iberian Peninsula). The
agreement increases if we take into account the uncertainty both in the
automatic classifier and in visual observations.
Key words. Meteorological and
atmospheric dynamics (instruments and techniques; radiative processes) –
Atmospheric composition and structure (cloud physics and chemistry)
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