Is there a downloadable archive of weather conditions in Europe?
Also, the temperature, humidity, precipitation by location?
So the database would enable, for example, to draw the chart with average temperatures in February or May in Berlin Or Paris.
The European Centre for Medium-Range Weather Forecasts (ECWMF) has a rather impressive data collection available via batch scripts, downloadable files or even tailored formats. The first two are freely available (under specific conditions) for non-commercial research but registration is necessary.
Just to give an example, the GRIB dataset lists 4115 parameters ... including your requested temperature, humidity, precipitation and location (latitude/longitude).
For more casual data retrieval, you might try weather services which focus on specific areas, such as Yr from Norway.
You should check out http://worldclim.org/. They have crunched a variety of data sets to produce average temperature and rainfall for each month of the year for most of the earth's surface at a sub-1km resolution. The data is downloadable as raster GIS files that you can load into PostGIS or desktop GIS packages. If you want the underlying data that they used to compile these averages, they have links to some of their sources here:
I've also packaged these up as an API if that's easier for your application:
http://www.datasciencetoolkit.org/developerdocs#coordinates2statistics
There are several related opendata.stackexchange related questions to weather in Europe. Here is a compendium of questions/answers:
Weather radar data for Europe, especially Latvia
Database with monthly climate/weather data by country
What data source for cloud coverage available with forecast and how to parse it?
How might I go about visualising historical temperature CSV data?
UK open data is available. See: http://data.gov.uk/dataset/metoffice_uklocs3hr_fc
hourly observations for approximately 150 UK observing stations, daily site specific and 3 hourly site specific forecasts for approximately 5000 UK locations.
There are a large number of datasets covering international areas at the U.S. National Oceanographic and Atmospheric Administration (NOAA). You can see some of the global daily indicators at http://www.ncdc.noaa.gov/most-popular-data#ghcn and elsewhere on that page. I think the Global Historical Climatology Network-Daily (GHCN-D) is the data you are seeking.
International indicators of climate change and data collected from many organizations are available at http://www.ncdc.noaa.gov/international
(Disclaimer: I am the Evangelist for Data.gov)
German climate data is available at the DWD.
For worldwide data of over 3000 professional weather stations from the german government authority (the DWD) i suggest you register for a (free) FTP account with them to pull it directly.
Although those are only 125 "free" stations in Europe, in particular for Germany it is the best-validated data available.
Forecast.io has a nice API for weather data.
https://developer.forecast.io/
Another nice resource if you want climate predictions is from the World Bank
I recommend OpenWeatherMap. There are more than 40.000 weather stations around the globe and an excellent coverage in europe. They provide a very good API for free and also 5 to 14 day forecast mechanisms.
try at weather underground. and here is a random search for historical data about Berlin http://www.wunderground.com/history/airport/EDDT/2014/9/20/CustomHistory.html?dayend=20&monthend=9&yearend=2015&req_city=&req_state=&req_statename=&reqdb.zip=&reqdb.magic=&reqdb.wmo=
