March 25, 2018

The SAWfPHY hygrometer integrated in the ZEPHYR gondola for STRATEOLE-2

On 18 January, Guyancourt's integration and testing platform began assembling the first scientific instrument in the gondola for the STRATEOLE-22 project. The SAWfPHY hygrometer is developed by the Dynamic Meteorology Laboratory.

Once loaded into the atmosphere by the STRATEOLE-2 pressurised balloons, the SAWfPHY instrument will measure the mixing ratio of water vapour near the equatorial tropopause.

It is indeed in this region of the Earth's atmosphere that the water vapour content of the stratosphere is determined. The humid air masses characteristic of the lower layers at these latitudes are transported by the vertical motions associated with the Intertropical Convergence Zone and eventually enter the stratosphere. This upward movement is accompanied by a cooling of the air masses, which gradually dries up.

Also, SAWfPHY (Surface Wave Acoustic Frost-Point HYgrometer) must be able to measure the very low humidity prevailing at equatorial tropopause temperatures that can reach -85 °C. Only 3 or 4 are molecules of water vapour out of a million molecules of atmospheric gas!

Nevertheless, water vapour is a potent greenhouse gas and several recent studies have shown that inter-annual variations in stratospheric water vapour content can significantly modulate global warming at the Earth's surface.

The instrument works on the basic principle of hygrometry with frost point. A sensitive element is cooled down to ambient temperature until a layer of frost appears on its surface. The measurement of the equilibrium temperature of this layer, called the frost point temperature, makes it possible to find the ratio of water vapour mixture in the air.

The main innovation of the instrument lies in the detection of the frost layer.

While conventional instruments use optical detection requiring a deposition thickness of the order of a few μm, SAWfPHY uses acoustic wave detection propagating on the surface of a quartz. The interaction between these waves and the frost deposition becomes measurable for a thickness layer 10 to 100 times smaller than with optical detection, enabling to optimise the performances of the instruments, especially the energy performance.

AIT's activities take place on the integration and testing platform installed at LATMOS at the Versailles - Saint-Quentin University site in Yvelines, a platform co-financed by CNES, the Île-de-France region and the agglomeration community of Saint-Quentin (en-Yvelines). The platform was inaugurated in June 2015.

The external instrumentation on the white box and the processing electronics that will be inside the gondola attached to the tripod.

Besides SAWfPHY, Claire Cénac, in charge of the instrument and Paul Monnier, manager of onboard software - LMD.


Philippe Cocquerez, Project manager of Strateole-2, philippe.cocquerez at

Albert Hertzog, LMD - Principal Investigator Strateole-2 and instrument SAWfPHY -  hertzog at