The collection of meteorological data at the release site was considered very useful in understanding the early stage of development of the plume. Two types of measurements were envisaged:
The first group included routine meteorological data such as wind speed and direction, temperature, humidity and pressure, and also micrometeorological information such as shear stresses and turbulent heat flux collected by means of a sonic anemometer and using the eddy-correlation technique.
In the second group the use of constant-volume balloons was planned to monitor the meteorological characteristics of the air masses responsible for the plume transport. Furthermore the use of the balloons could provide real time information on the trajectory of the plume for approximately a hundred kilometres downwind.
RISØ National Laboratory (Denmark) proposed the ground micrometeorological measurements while Paul Sabatier University (France) and the Swiss Meteorological Institute performed the constant volume balloon launching and tracking. The latter also offered to perform SODAR (SOnic Detection And Ranging) measurements. This ground-based remote-sensing technique provides information on vertical profiles of average horizontal and vertical wind speed. Radio-sonde launches were also planned to complement the data collected by means of the SODAR, which extends its range only to a few hundred metres in the vertical. METEOFRANCE offered the instruments and the expertise for this type of measurement.
Other than the meteorological measurements collected at the release site, ten French nuclear power stations offered SODAR measurements routinely collected at their sites. Furthermore the European Centre for Medium Range Weather Forecast offered the information collected by the stations covering the area from 35° to 70° N and 40° E to 30° W by means of radiosoundings during the tracer experiment. The ECMWF analysed data concerning the period of the release also represented a valuable contribution.