English: This beautiful panorama of ESO's Paranal Observatory was taken on 5 July 2012, and marks one of the driest days ever recorded at the Very Large Telescope complex. Paranal sits like an island in the middle of the frame, with massive cloud banks floating below, over the distant Pacific Ocean.

The extremely low humidity at Paranal during this period was recorded by a water vapour radiometer known as LHATPRO, which monitors the atmosphere to support the observations carried out at the observatory [1]. Meteorologists from two Chilean universities identified the cause for these unusually dry conditions: high-altitude Antarctic air moving far to the north, and descending over Paranal.

This cold front lingered around Paranal for over 12 hours, causing a record-low level of humidity in the air above the observatory [2]. Florian Kerber (ESO) and colleagues analysed this unusual weather, publishing the results in a paper in the Monthly Notices of the Royal Astronomical Society on 29 January 2014, available here.

So, a dry desert… what is so special about that? Well, dryness of this magnitude is normally experienced at much higher altitudes, for example at the ALMA Observatory on the Chajnantor Plateau, which is located at 5000 metres above sea level — at 2635 metres, the altitude of Paranal is around half of this. Given that infrared observations can be best taken when there is little water vapour in the air, this could mean that routine monitoring using the LHATPRO radiometer will give astronomers the opportunity to exploit future dry spells at Paranal, to obtain great infrared observations of the Universe around us.

The photo was taken by ESO photo ambassador Gabriel Brammer, who coincidentally experienced the sunset that immediately preceded this dry spell, and found it to be extraordinarily clear and beautiful. Gabriel works as an astronomer at the ESO La Silla-Paranal Observatory. When not supporting the operations of the observatory, he studies the formation and evolution of distant galaxies using the most sophisticated telescopes and instrumentation in the world, including the ESO Very Large Telescope and the Hubble Space Telescope.

Notes

[1] The Low Humidity and Temperature Profiling radiometer (LHATPRO), manufactured by Radiometer Physics GmbH in Germany, uses strong spectral lines from certain elements to measure the water content of the atmosphere.

[2] The humidity is measured in the form of precipitable water vapour — a measure of atmospheric water content. It is the amount of water in a column of the atmosphere if it were all to fall as rain. In this case only 0.1 mm of precipitable water vapour was measured — much less than the usual (but already low) figure of 2 mm at Paranal.