Before I arrived in Spain on a "shopping" trip, looking for a suitable house I did a lot of online research into the broad areas that I should focus on.
My top priorities were to avoid light polluted areas and to maximise the number of cloudless nights. I found it very hard (actually, impossible) to get information about the number of cloud-free nights in Spain. However, I made the assumption that areas which had large values for hours of sunlight would also be likely to have many cloud-free nights, too.
As you would expect, holiday resorts are good at promoting the amount of sunshine they get - but I knew that resort locations would not qualify due to the light pollution they suffer, as well as being far too expensive for my house buying budget.

Inland areas are more concerned with crop-growing and farming than with leisure activites and therefore do not publicise sunshine records. I did find out that the province of Almeria contains Europe's only desert. This sounded promising, as did the fact that there is a joint German-Spanish observatory in the region.

The effect of light pollution is to make the sky brighter over the lit-up town. This can be seen from a distance as a "dome" of light on the horizon, although from inside the town itself, peoples' eyes never adapt to true darkness so they will not realise that the sky is not as dark as it should be. The only affect they may notice is the lack of stars.
From an astronomical point of view, the increased brightness of the sky means that a lot of the dim objects are washed out or even invisible. It's similar to listening to music with a pneumatic drill digging up the road just outside your window; you can still hear the loud bits, but all the nuances disappear.

What I wanted to know was which areas of Spain had the worst light pollution and which areas are still relatively free. A lot of work has been done by groups like this to map the areas in europe that are affected by light pollution. I took their map of light polluted areas and overlaid it with a map of the major towns and cities in Spain, see the map on the left. As you can see, there is a good match (once you take into account the different perspectives of the two maps) between major towns and major light pollution.

The colour coding shows sky darkness, ranging from the darkest areas in grey through blue, green, yellow and with the worst polluted areas as white.

I already knew from my research into sunshine/clouds that the south-eastern corner of Spain looked quite promising. The light pollution plots showed that there was a light-free pocket inland from the city of Almeria, stretching east.
See the zoomed in map on the right.

Based on this information, I started looking in the area centred on a small town called Lubrin.

This made me reconsider my approach and I realised that the reason this area looked so good on paper meant that there were practical drawbacks to living in a sparsely populated area.

I changed my search area to one of the other pockets of blue on the map. This time looking around the area to the north of Baza. I struck lucky by hooking up with an estate agent who "got" my requirements. Apart from showing me the house I eventually bought, he also showed me a place quite close to the Calar Alto observatory (see above). While this was a great location, it did suffer from the drawbacks of being literally in the middle of nowhere. I was concerned about being this cut off - there were no neighbours or even a mobile phone signal, let alone a landline. This could be a problem if there was ever an emergency.

While the light pollution map is a good indication of large-scale effects it is not designed, nor able, to tell you how good or bad a particular point will be. For that there is no substitute for going and checking a location yourself.

In practice a person with good eyesight can see stars as dim as magnitude 6, provided the sky is dark enough that stars this dim aren't blotted out by light pollution. Astronomers describe the darkness of the night sky in terms of the dimmest stars they can see, using just their eyes. For example in the middle of a town, excess light means that your eyes never get fully dark-adapted and therefore never reach their greatest sensitivity. For that reason, and the consequent lack of contrast between the bright sky and the dim stars, the dimmest stars that a person can see may only be magnitude 2 or 3. We would say therefore, that the sky has a limiting magnitude of 2 or 3.

Because these measurements are dependent on individuals, they can be somewhat unreliable. A person with good eyesight can see dimmer stars that someone who's eyes are not so good. Another factor that comes into play is how experienced an observer is: for example a veteran observer can use "averted vision" by not looking directly at a star, to improve their eyes' sensitivity.
Fortunately it is now possible to get an instrument that removes the subjective element from sky quality measurements and can prevent arguments about just how good or bad a particular location is.

Using one of these meters, I measured the sky brightness at Cuevas del Negro to have a limiting magnitude of 6.3. To put this into perspective, at my house in Marlow, the limiting magnitude on an average night is 3.5 - on a good night it sometimes gets down to 4. Here are two photos taken with a fisheye lens that shows almost all the sky. The one on the left is a 60 second exposure taken on a dark night from a field a couple of miles from my house in Marlow. The photo on the right is the same exposure time taken at Cuevas del Negro.

Summer night, about 1:00 a.m. Note faint view of the milky way running across the image	Winter view, north to the top. Orion visible in lower quarter of image