La Silla Site Information

Weather and Dome-Closing Conditions

Should weather conditions, e.g. high wind or humidity, hamper operations, observers are expected to follow the instructions from the Telescope and Instrument Operators, which are to be followed unconditionally. The closing conditions to be respected are given below, but TIOs are entitled to close at any time if they consider that the conditions are dangerous.

For further information, go to the La Sllla Meteo Monitor help page.

Wind Speed

A master anemometer records wind speed in the telescope area. Their readings are displayed on the MeteoMonitor web page. The TIOs will monitor this information and take the corresponding actions. The wind restrictions are summarized in the table below.

Wind Speed Monitor Light Codes
Speed (m/sec)
Action By Observer
< 14
14 - 18
Don't observe into the wind
> 18
CLOSE DOME: Don'wait to be told!. After 30 minutes below 16m/s open again

Wind Speed Monitor Light Codes

:These rules apply for all ESO La Silla telescopes excepting the NTT for which the speeds are 12 and 15 m/sec. respectively.   


General humidity sensors are installed on the weather tower; their readings are relayed on the MeteoMonitor . Domes have to be closed when the relative humidity exceeds 80%, and can be re-opened when it remains below 70% after 30 minutes. Similarly, the domes must be closed when the temperature difference between the coldest part of the telescope and the dew point drops below 3.5 degress. It is the responsibility of the TIOs to decide when to close and re-open the domes. It must also be noted that, because of water, snow or ice build-up on the dome, it can happen that the TIO will not re-open even if the conditions seems good. The TIOs' instructions have to be followed unconditionally.

Meteorological Information

Further information about the prevailing and future weather conditions on La Silla can be obtained via our Home Page. In particular, the MeteoMonitor displays a continuously updated information about seeing, wind-speed and direction, humidity, atmospheric pressure, etc.