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Measuring equipment and procedure

The measurements were taken in the upper part of the ESO 3.6-m dome (fig. gif), taking advantage of the bridge crane there located, during the evening of a windy day when the dome anemometer indicated almost constantly a mean velocity of 18 m/s from North. The dome slit was entirely opened.

A vortex type anemometer (fig. gif) was utilized, which is particularly suited for fast response measurements, having a resolution of 6 mm. The measurements consisted of wind velocity sequences of 137 seconds each with 4096 records, therefore at the frequency of nearly 30 Hz. Several such sequences were recorded at different positions along the path of the bridge crane as shown in fig. gif. Before each sequence, the dome was rotated forth and back in order to find, rather empirically, the azimuth angle at which one would have the stronger feeling of wind flow and turbulence. Not surprisingly, this was found to be approximately facing the mean wind direction. Therefore the values measured are properly worst case quantities, as one may expect that during observation the slit would be facing the wind only a fraction of the observing time.

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995