itoworld.com has published this amazing visualisation of the Northern European airspace returning to use after being closed due to volcanic ash. Due to varying ash density across Europe, the first flights can be seen in some areas on the 18th and by the 20th everywhere is open. The data is a mashup from flightradar24.com, openstreetmap.org and contributors.
I am still wondering why there were so few flights to accumulate data to confirm or falsify the data of the models that predicted the ash concentration over Europe and were the basis for the airspace lockdown. I only heard of one flight over Germany. Do we know the data was right?