hertz (Hz)		Heinrich Rudolf Hertz (1857–1894)

Heinrich Hertz was a German physicist. One of his professors, Hermann von Helmholtz, suggested that he should investigate electromagnetic waves, and in 1887 Hertz began experiments with radio waves. His experiments laid the foundations for radio communications and radar, including meteorological radar. Indeed, radio waves were first called 'Hertzian' waves.

Hertz was also a pioneer in the study of moist thermodynamic processes in the atmosphere. Kelvin in around 1862 had given the first correct description of the saturated adiabatic process in rising air in the atmosphere. Such problems lend themselves well to treatment by graphical methods, and in 1884 Hertz published a diagram for calculating atmospheric adiabatic changes in the journal Meteor. Zeit. He started with a parcel of moist air, subjected it to lift, and considered the adiabatic process of the rising moist air to be composed of four stages: dry air, cloudy air and rain, hail, and ice crystals and snow. At first the rising air remains dry, but eventually as the temperature drops (due to its expansion at lower pressures) the dew point is reached, and further rising and cooling results in condensation with cloudy conditions and rain. Continued rising takes the air to higher levels and colder temperatures and the raindrops freeze to form Hertz's 'hail', while still higher and colder conditions result in the fourth stage of ice crystals and snow.

Hertz's diagram was the first atmospheric thermodynamic diagram and the precursor of all modern diagrams such as the emagram and the tephigram (McDonald, J.E., 1963: Early developments in the theory of the saturated adiabatic process. Bull. Amer. Meteor. Soc., 44, 213-211).

To honour his scientific work, Hertz' name was given to the SI (International System of Units) unit of frequency. One hertz is equal to one cycle per second. The hertz unit was not named after Heinrich Hertz' nephew, Gustav Ludwig Hertz (1887–1975), a 1925 Nobel Prize winner in physics, who has also been honored on many stamps.

¹Hertzian waves are electromagnetic waves produced by oscillations in an electric circuit, as a radio or radar wave: first investigated by H.R. Hertz; "Faisceau Hertzien" or "Faisceaux Hertziens": antenna-based wireless transmission and reception of electromagnetic signals.

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