THE clouds and the pressure systems that collectively provide our daily weather are steered in one direction or another by the flow of winds in the upper atmosphere. However, as any casual observer of a drifting cumulus can note, for much of the time upper levels winds differ significantly in speed and direction from those at the surface. From the earliest days of meteorology, therefore, accurate forecasting has been crucially dependent on information about the upper reaches of the atmosphere.
Our predecessors were ingenious in their efforts to acquire this information. Some; did it the hard way, by ascending personally in hot air balloons with their thermometers and barographs - and the lucky ones survived to use the data. Others used recording instruments attached to kites, or fixed them to free flying hydrogen filled balloons in the hope that some kind person would return the equipment with its readings from the landing place. The break through for "real time" upper air observations came, however, with the discovery of radio, which allowed the development of radiosonde.
The radiosonde comprises a small box, made of a very light material, which contains compact instruments and a radio transmitter. The package is attached to a hydrogen filled balloon, and as it floats upwards the instruments measure the temperature, pressure and humanity of the surrounding air, the instruments in turn are linked to the transmitter in such a way that the frequency of the transmitted signal varies with different values of these elements. The radio signals are picked up by a ground station for conversion back to degrees, hectopascals, or percentage relative humidity.
The height of the radiosonde, and the speed and direction of the wind at various levels, are obtained by observing the movement of the balloon itself. It drifts with the wind as it ascends, so successive "fixes" at, say, 60 second intervals allow the operator - or now, more usually, a computer - to calculate the wind speed and direction at many different levels in the atmosphere. Until comparatively recently these measurements were obtained by attaching a metal reflector to the balloon, and tracking it with radar, nowadays the navigational systems normally used for ships and aircraft can do the job with much less fuss.
Satellites provide a wealth of data over a vast area of the globe, but the information they provide is somewhat "blurred" in the vertical; it is difficult to relate their temperature and humanity values to a specific height. The radiosonde, however, along with providing highly accurate data over a particular spot, also gives readings from which the satellite data over a wide area can be properly calibrated.