One hundred years ago, Leon Teisserenc de Bort made his "most striking discovery" of the tropopause, as described here yesterday in Weather Eye, by sending recording instruments aloft attached to hydrogen-filled balloons, the data from which, when recovered, were analysed in the succeeding days, or even weeks.
When it came to actually predicting the weather, however, such methods were of little practical use. Weather in formation, be it near the surface of the Earth or from the upper atmosphere, is a highly perishable commodity and, to be of benefit to the forecaster, it must be to hand "real-time".
A solution came some 60 years ago with the development of the radiosonde. As its name implies, the radiosonde depends on radio; it consists of a small box containing the required meteorological instruments and a transmitter. The package is attached to a balloon and, as they float upwards, the instruments measure the temperature, pressure and humidity of the surrounding air.
These are linked to the transmitter in such a way that the frequency of the transmitted signal varies with different values of these element, and the radio signals picked up by a ground station can be converted back into "degrees" or " hectopascals".
Nowadays, of course, satellites provide a wealth of data of this kind - but they have their limitations. A satellite picture covers a vast area of the globe, but at any particular spot the information it provides is somewhat "blurred" in the vertical.
It is difficult to relate temperature and humidity values to a specific height, so the radiosonde is still an indispensable part of the observing sys tem when very accurate readings are required over a particular location.
Indeed, some of you may have seen these instruments soaring into the midday sky; they are launched several times a day from Valentia observatory in Co Kerry and from the only other upper-air station on this island in Co Down, not far from Belfast.
Balloon ascents like these also provide information about the wind at upper levels in the atmosphere. Until comparatively recently, this was achieved by fitting the balloon with a radar reflector and then tracking the balloon by radar.
Since the balloon drifts with the wind as it ascends, successive "fixes" at, say, 60-second intervals allowed observers on the ground to calculate the wind speed and direction at many levels in the atmosphere.
Today, modern radiosondes use satellite navigation systems to pinpoint their position at various points upon their journey upwards, and the wind data at various levels are calculated with great precision by computer.