AS we noted yesterday in "Weather Eye", meteorologists in their constant strife to build a picture of the atmosphere in three dimensions depend a great deal on a little gadget called a radiosonde.
It consists of a small package of instruments and a radio transmitter carried aloft by hydrogen filled balloon. As the balloon floats upwards, readings from the instruments are relayed to ground as radio signals, which are then translated back into degrees, hectopascals, or percentages of relative humidity.
In addition, the motion of the balloon itself, tracked by one of several aeronautical radio navigation systems, provides data from which the winds at different levels in the atmosphere can be computed.
Weather people have always been conscious, however, that there is another potential source of information of this kind. An ordinary passenger carrying aircraft landing or taking off traverses the atmosphere from the surface up to perhaps 30,000 feet, and suitably equipped should be a valuable source of weather data.
Traditionally, gathering this information was a complex matter. A crew member aboard the aircraft had to read the instruments and transmit the information by radio to the nearest ground communications centre there it was transcribed on to a standard pad and "keyed in" to the international meteorological communications network.
In such a complex chain of events, occasional errors and omissions were inevitable, and even at best it was a mundane, uninteresting and largely thankless task for all involved.
ASDAR is not a much more efficient way of achieving the same objective. The acronym stands for Aircraft to Satellite Data Relay, and the equipment comprises a small "black box" fitted inside the aircraft.
It communicates with sensors on the airframe which measure the temperature of the air and the barometric pressure, and it is also connected to the aircraft's navigational system which, combined with other data from the aircraft, allows it to estimate the speed and direction of the wind.
In level flight ASDAR reads the temperature and pressure and estimates the wind every seven minutes on ascent or descent, readings are taken more frequently, in order to provide a picture of the vertical distribution of these elements in the atmosphere, supplementing data already available elsewhere from radiosondes.
This information is stored in the system's memory, and every hour it is disgorged to the nearest geostationary satellite, together with data on the aircraft's position and height and any turbulence experienced.
From the satellite the information is routed immediately back to Earth and straight into the Global Telecommunications System used by meteorologists, all without human effort or intervention.