Weather Statistics

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.
Cloudy days in Cuevas del Negro

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.

cloudy per month in Cuevas del Negro

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!.

clear nights in Cuevas del Negro