The development of the radio and the relative miniaturization of transmitters enabled a decisive step to be taken in altitude measurements. In 1927, the Frenchmen Pierre Idrac and Robert Bureau received, on the ground, for the first time, a short-wave transmission coming from the stratosphere. In 1929, Robert Bureau invented the radiosonde. This device, transported to a high altitude by a balloon filled with hydrogen, measured the pressure and temperature of the air and simultaneously transmitted the two measurements by radio.

Pierre Idrac (1885-1935), photographed in 1928 in the balloon shed of Trappes observatory (Photo Météo-France)

The world meteorological radio-sounding network. In red, the surface sounding stations, in blue, the soundings from ships

Nowadays, this system has become widely used. Helium has replaced hydrogen, which is too dangerous. The radiosonde also measures the humidity of the air. Its movement is tracked by a radio-positioning system (Loran C or GPS), which enables the speed and direction of the wind to be calculated throughout the balloon's climb upwards. Twice a day, at midnight and noon UTC, in Trappes, Nancy, Brest, Bordeaux, Lyon, Nîmes and Ajaccio and in the French overseas departments and territories, in France as in all countries in the world, a balloon equipped with a radiosonde is released. After a one and a half hour climb to a height of 30,000 metres, the balloon explodes and falls back to Earth just a few tens of kilometres from its departure point.