Measuring instruments

To fulfil its different missions, the weather station uses three types of tool: measuring instruments, which vary depending on the altitude, detection/immediate forecasting equipment and analysis equipment in terms of general forecasting.  However, the human eye remains the simplest and foremost instrument for immediate observation.  A meteorologist's first reflex has always been to raise his eyes to the sky and this should under no account be seen as a sign of weariness or irritation!  As an alternative he can always climb up into one of the station's observation towers.

At the entrance, the Sodar acoustic sounder emits its stridulations in order to measure the direction and strength of winds in the lower atmospheric layers.  Based on the Doppler effect (backscattering of sound waves by the atmosphere), it can take measurements up to an altitude of 500 m and a radius of one kilometre.

For low-altitude measurements, three weather masts take readings up to 100 m in altitude.  They are installed at the ELA (Ariane Launch Complex), the Sounding Rocket Launch Complex and, before long, the Soyuz Launch Complex.

For measurements at 10 m in altitude, four automatic stations installed at the weather station, Ile Royale, Kourou Beach and Pariacabo determine temperature, pressure, humidity and wind.  The above three items of equipment are all French-built.

The Sodar system is next to the sounding balloon facility, whose high-altitude measurements are essentially used to ensure flight safety: measuring wind pressure, atmospheric pressure, temperature and humidity. Two kinds of balloon are used: the Jimsphere (named after its American inventor) and the Japanese Totex balloon.  Their measurements are analysed by the Digicora system (Finnish built).  The Jimsphere resembles an aluminium sea urchin, as it is covered in spikes which are effective in tracking the wind.  It flies as high as 18 km in altitude, tracked by the Bretagne 1 radar on the ground.

The principle of the Digicora system is relatively simple: a probe fitted with a GPS is suspended from a large helium balloon.  At a rate of around 300 m per minute, it takes two hours to climb to a minimum altitude of 30 km before bursting.  The balloon is manufactured from a special dilatable latex.  On the ground it measures 2 m in diameter, whereas when it reaches its highest point this has increased to 6 or 7 m.  This enables it to maintain the balance between the masses of air (whose pressure reduces with altitude) and the helium.  A cubic metre of air on the ground weighs a kilogramme, whereas it only weighs 10 g at the point where the balloon bursts, at 30 km in altitude.  However, latex does have one major drawback: it becomes as brittle as glass at low temperatures.  This was observed during a flight in May 2006, when the balloon burst at only 18 km in altitude, when confronted with an external temperature of -82°C.