In 1879 he returned to Magdeburg to practice medicine. In 1885 he earned a doctorate in secondary studies at the Faculty of Philosophy at the University of Halle, and subsequently became a scientific officer at the Royal Meteorological Institute at Berlin-Grünau. From 1905 to 1914 Assmann was director of the Prussian Royal Aeronautical Observatory at Lindenberg, and afterwards was an honorary professor at the University of Giessen.