I often need to calculate heat balances for large biogas plants, an important factor is outside temperature. I normally use the weather data from Klimadiagramme.de where I can find the mean monthly temperature and precipitation. This allows me to calculate the heat demand of my plant for a given month reasonably well. However, I also need to know extreme weather. I'm not sure what the metric here is, but I'm not looking for "the coldest night in the last 100 years" but "this was the coldest average over 2-3 days in the last x years" or something similiar.
So, what is a good, free source of climate data that gives me information about extreme weather events as described?