Earth's atmospheric circulation pattern requires westerly meteorological observations for prediction of weather conditions to the east. Weather observation stations in Greenland improved the accuracy of weather forecasting for the Atlantic Ocean and northern Europe for tactical advantage in the Battle of the Atlantic and European theatre of World War II. Greenland had been part of the Danish colonial empire since 1814. Greenland appeared relatively unprotected following German occupation of Denmark on 9 April 1940. The Allies of World War II became concerned about the possibility of Axis military bases on Greenland. The cryolite mine at Ivittuut was a strategically important source of flux for electrolysis of aluminum ores by the Hall–Héroult process for aircraft production.