Number of clear nights at Cuevas del Negro, Andalucia
The other important question about any observing site, apart from how dark
the sky gets, is how many nights of the year is the sky clear?
The nearest large town, Baza, has a weather station run by the Andalucia government. I have dumped it's weather data [link] since August 2000 in an effort to estimate the number of clear and sunny days each month.
The chart on the left shows my efforts in this pursuit, although the data is by no
means easy to interpret. I have based the estimate of the number of cloudy days
on the amount of sunshine the weather station measured each day. This is
a measure of the total sunshine energy in MJ per square metre - rather than
the simpler number of hours of sunshine. As a consequence, the figure varies
throughout the year as the sun is higher (and therefore more intense) during
the summer than during the winter.
From the above it is no surprise that the clearest days are during the summer, with very few cloudy (or rainy, for that matter) days. The next chart shows the breakdown by month more clearly.
The next plot might be a bit of a stretch. Looking at the number of days
each year that got two-thirds or more of the maximum sunlight (i.e. were "sunny"
days). If we presume that the nights were equally as clear, then the number of
nights when some observing would be possible is going to be about the
same. So, plotting the number of sunny days as being the same as the number of
"observable" nights - excluding such factors as the full moon, gives the graph
on the right.