The ASM during UT1 Science Verification

Astroclimate Highlight during VLT-UT1 Science Verification

When at one of the best observatories worldwide, over two weeks and more, the sky is often cloudy, the seeing poor, the wind fairly strong and blowing from unusual directions, one is allowed to start talking of an astroclimatological anomaly. When this occurs during the science verification of a telescope using the largest monolithic mirror ever built, the event receives a particular attention which justifies attempting a more detailed analysis.

Cloudiness at Paranal is the cunjunction of seasonal trends and El Nino events on top of some longer yet unexplained cycle (The Messenger 90, 6). As we are currently in the lows of the latter cycle and despite the end of the 97-98 El Nino event, August 1998 was promising less than 70% photometric nights: the two nights lost for cloudiness during the two weeks of science verification were thus perfectly expectable. The wind at Paranal is stronger in Winter (30% of the time more than 10m/s) than in Spring or Summer (15% only): one and a half nights lost because of wind in two weeks of observing is thus not anomalous. As clouds have no reason to prefer windy nights, the two previous effects tend to cumulate and the total time lost was close to 25% of the total available observing time, nothing to be ashamed about!

Unfortunately, the seeing conditions were not at all inside the statistical margin as can be seen on the Figure: not only did we have an excess of very bad seeing (10% of the time worse than 2 arcsec) but also a deficit of good seeing periods (3 times less than normal).

This situation cannot be explained by a synoptic analysis: only a slight excess of temperature was reported over South-America during the first week, the jet stream behaviour was quite normal as well during the whole period. Nevertheless the wind vane at Paranal more than ususal kept pointing at North-East or South-East where the bad seeing comes from (valleys and nearby summits). In addition, a cold front causing a sudden drop of the air temperature turned the ground around the observatory into a really efficient local seeing generator several degrees warmer than normal.
Whatever further improvements we make in the understanding of the generation mecanism of atmospheric turbulence, the operation strategy of ground-based astronomical facilities has nevertheless to be designed to confront from time to time a highly non-deterministic environment.

Night by Night ASM-DIMM Seeing Monitoring

Send comments to <Marc.Sarazin@eso.org>
Last update: September 8, 1998