How are airplanes de-iced in the air?

If you’ve flown in cold weather, you’ve probably looked out your window before take-off and seen vehicles circling the plane, spraying de-icing fluid on the wings. Preventing ice formation is not only crucial in the winter months, but also in the summer months. Indeed at higher altitudes, the temperature is well below freezing point all year round. So how does it work in the air? To learn more about it, listen to this radio interview ( given by Dr. Elmar Bonaccurso. [Only in German]