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 this I have counted a day as "cloudy" (i.e. having some proportion of the
maximum possible sunshine blocked by clouds) if the total intensity for a day
is less than 66% of the best day that month. That assumes there is at least ONE
clear day that sets the baseline. While this does not mean that a day is
completely clouded over, it gives some idea of how many days per month had
more than a third of their sunshine blocked. This still allows for
the other two-thirds of the day to be in pure, brilliant sunshine, so the data looks
more pessimistic than it probably is. It also only tells us about the daytime
conditions, while we're primarily interested in how clear the sky is at night.
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.
So even during the winter there's a reasonable chance of clear skies. I
have visited Cuevas del Negro during December and enjoyed 4 or 5 clear nights in a single week.
That's observing from mid-evening to well past midnight, - although this is by no means guaranteed and I have also experienced the opposite: very few clear evenings during a week long stay.
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.
I must emphasise that this is a calculated number, not an observed one - and
is based on an arbitrary choice of two-thirds as the difference between a clear
and cloudy day. So it shouldn't be taken as definitive. Just as an idea of how
much observing could be done in the course of a year. From this, it appears
that 2005 was a very good year!.
