Julius von Hann (1839-1921) was an Austrian meteorologist.
In signal processing, the Hann window is a window function named after him by R. B. Blackman and John Tukey in "Particular Pairs of Windows." published in "The Measurement of Power Spectra, From the Point of View of Communications Engineering", New York: Dover, 1959, pp. 98-99.
In the aforementioned article, the use of the Hann window is called "hanning", e.g. "hanning" a signal is to apply the Hann window to it. This has caused a lot of misconceptions about the name of the Hann window, leading many to call it the "Hanning window".