I have put some data in Excel and analysed the climatic conditions in Maastricht, the Netherlands, over the period 1957 – 2007. Added: sunshine duration comparison between Maastricht and Madrid.*

Temperature and precipitation
In the two graphs below you will find the average temperature and precipitation over the last 51 years. The light blue bars depict the maximum temperature for a specific month, while the dark blue bars show the minimum temperatures. As you can see, the average minimum temperature in winter is close to zero, while in summer the maximum average temperature is about 23 degrees Celsius. The data show a higher variability around the averages in winter than in summer.

The amount of precipitation is equally spread over the months; there are no extreme wet or dry periods. The dark red bars depict the average precipitation per month, while the light red bars show the variance around this average. As you can see April is not as wet as the other months. The summer and winter periods are the wettest. The variance follows the pattern of the averages, i.e. higher averages come with higher variances. In practice this means that e.g. August is the wettest month of the year on average, though it is likely that there were years with nearly no precipitation and years in which the level of precipitation exceeded the average substantially.

Also note the similarity of variances in April and November. It is more certain that November will be wet as the average is high and there is a relatively low spread around the average. In April the opposite hold true: although we might expect a relatively dry month, over the years this has fluctuated relatively strong.

During the last 51 years average precipitation equalled 763,8 mm for a full year, with a variance of 136,9 mm. It is not uncommon to see yearly precipitation levels between 625 mm (763 – 136) and 900 mm (763 + 136). The yearly, average minimum temperature equalled 6 degrees Celsius, while the maximum stands at 13,8 degrees.

Global warming?
According to experts global warming should bring higher temperatures and more precipitation to the Maastricht area. Well, below you will find the graphs depicting the average mean temperature (which is the average of the all the daily average temperature readings during one year) and the total precipitation per year.

It seems that the last decade has been warmer than the forty years before. Below a similar graph for the yearly precipitation. Please not that axis on the left does not start at zero, thereby visually amplifying the findings.

It seems rather difficult to tell if the last decade has been wetter than the ones before.

Sunlight
Although precipitation does not fall from a clear sky, the amount of it does not give a definite indication of greyness of the sky. The meteorological institute provides two more indicators, i.e. the duration of daily sunshine with respect to the total possible sunshine for that day and cloud cover. The cloud cover is expressed in octaves (eighths). The cloud cover is therefore graded on a scale from 0 (no clouds at all) to 8 (completely cloudy). The values in between represent an increasing level of clouds (going from 1 to 7). Below you will find the sunshine per day (blue, on the left axis) and cloud cover (red, on the right axis).

We can observe that the percentage of sunshine is far lower in winter (20%) than in the summer (40%). Although this sounds very logical, keep in mind that it is a percentage of the maximum daily sunshine possible. In winter the time between sunrise and sunset equals roughly 8 hours, while in summer this number is doubled to 16 hours. So, in winter the sun shines 20% x 8 = 1.6 hours, while in summer the sun spends more time on earth, namely 40% x 16 = 6.4 hours. This is four times the duration of sunshine in winter. The values for the cloud cover correspond inversely with the numbers for the sunshine percentage. There are more clouds in winter than in summer. From a light perspective August is the most lucid month of the year.

Concluding
In the Maastricht area there is no extremely dry / wet period. The winters and summers are somewhat wetter than the spring and autumn periods. As the variance in summer is higher than in winter, it is more likely that in summer precipitation levels differ per year, while in winter monthly levels of precipitation are similar. Temperatures will be fluctuating the most in winter. Furthermore, winters are relatively dark as cloud cover increases (without leading to higher levels of precipitation).

Sunshine duration comparison Maastricht and Madrid
Below a comparison using data* from the KNMI and Agencia Estatal de Meteorología over the years 1971-2000. I assume that the definitions of sunshine duration are comparable.

It goes without saying that Madrid is far sunnier than Maastricht. The annual average amount of the precipitation equals 436 mm. This is some 325 mm a year less than in Maastricht.

* Data were obtained from the website of the Royal Netherlands Meteorological Institute (KNMI) and from the Spanish Meteorological State Agency (Agencia Estatal de Meteorología).