Who benefits from the forecasts ?

When the launcher is being transferred from one building to another, this results in almost 500 tonnes of solid propellant being placed on a table in the open air, surrounded by vital infrastructures.  Because of its size and shape, a launcher makes a very good lightning conductor and thus risks attracting lightning strikes from the sky!  This is exactly what happened at the Kennedy Space Center in 1987, when an Atlas-Centaur launcher was struck by an enormous bolt of lightning.  Moreover, even if the dangers involved are less serious, a launcher on its table is exposed to violent, potentially destabilising gusts of wind.

Everyday weather forecasts provide all the information concerning lightning, rain and wind that the base needs.  They are used, for instance, when works are carried out at a great height, such as for construction or cleaning operations, where the technicians rely for their personal safety on stable meteorological conditions.

Pre-launch, the weather unit keeps the key players informed: CNES CSG management, which gives the authorisation to launch, and the Flight Unit which takes the decision whether or not to launch.  The DDO (Director of Operations) requires weather (C1, C2 and C3 criteria) and wind criteria for the launchpad.  In order to mitigate any toxicity hazards, Ground Range Safety needs information about wind direction and speed, both on the ground and in the lower layers of the atmosphere, as well as the risk factor for rain (see the Encounter section).  This information is also used by the Rescue Director (Subdirectorate for Range Safety and Protection of the Environment), who consults wind measurements taken at the beach.

Concerning the management of toxicity hazards, wind measurements seem to dominate here too, especially those taken on Kourou Beach.  This is quite simply because a launch cannot take place if wind speeds are higher than a threshold calculated by Range Safety (between 5 and 10 metres per second, depending on the launch), so that in the event of an accident involving the launcher, any toxic hazard would not breach the danger zone perimeter (thus protecting inhabited areas).  This was a particularly vital issue with Flight 171, where between T-1 hour and T-22 minutes the winds on Kourou Beach were not favourable.  Similarly, wind measurements taken at altitudes between 0 and 30 km enable the flight safety service to simulate where debris would fall in the event of an explosion, thus allowing them to prohibit the launch if necessary.  These are the range safety constraints imposed by weather conditions.

In any case, the Director of CNES/CSG, as the guarantor of safety at the base and protection of people and property at Sinnamary and Kourou, has the final say about whether or not to authorise a launch.

Post-launch: once the launcher has left its launchpad, things can really get complicated.  While speeding through the atmosphere, it is exposed to high-altitude winds.  It must therefore constantly adjust its trajectory, depending on the strength of these winds, in order to maintain the correct course.  The greatest risk, however, remains that of lightning strikes, this time in flight.  Acting just like a lightning conductor, the launcher can trigger lightning while traversing certain clouds, due to the phenomena of friction and electrostatic polarisation.  Its pointed nose and high speed cause the charge in the cloud to condense suddenly, even if the cloud alone normally poses no risk.  This is particularly true with cumulus congestus clouds, which range from 5,000 to 6,500 m thick, although the most treacherous are the well-known cumulonimbus, which can be 18 km thick over French Guiana.  Highly charged with static electricity, due to friction between their hailstones and positively- and negatively-charged water droplets, these clouds are very inclined to generate lightning.  In the hours following the launch, weather readings are transmitted to the DLA (Ariane Launcher Directorate), EADS and Arianespace.  The latter then proceed to 'analyse' the flight, in order to try and correlate the information with that received from the launcher each time it modifies its flight trajectory.  Because these constant readjustments involve a great deal of effort on the part of the launcher, this information can help to reduce the number of readjustments for future launches.

The major works currently underway at the Guiana Space Centre testify to the impending arrival in service of Soyuz. A decision was taken to create a special working group, devoted to coordinating and verifying the suitability of weather missions for Soyuz using the planned equipment.  This working group will study the following aspects, based on a GTO (geostationary transfer orbit) mission:

* the Launcher authority's Expression of Needs, as well as those of the operator and Range Safety, in terms of meteorology, for a Soyuz campaign;

* the summary of these needs for each phase of the campaign, the modified organisation and the ensuing weather mission;

* the ability of the planned equipment to fulfil the mission;

* the technical, financial and scheduling consequences of modifying this equipment, if deemed necessary.