The results of the original site campaign testing carried out at Fuente Nueva (near the current site of the JKT) for twelve months in 1974-1975 showed that the Observatorio del Roque de los Muchachos is one of the best astronomical sites in the world. Meteorological observations made at every hour of the night indicated that during the testing period 59% of dark hours were photometric and 71% spectroscopic. A photometric hour was defined as an hour within astronomical darkness for which the relative humidity was less than 90 per cent, the average wind speed less than 15 m/s, the zenith extinction in V less than 0.3 magnitudes, and obscuration by cloud less than 5 per cent above 30 elevation and less than 15 per cent above 10 elevation. Hours meeting the above humidity and wind speed restrictions, but with zenith extinction less than 0.5 magnitudes and obscuration less than 50 per cent above 30 elevation, were classified as spectroscopic hours. Other hours were termed unusable.
Analysis of Polar Trail films showed that for 40 per cent of usable hours the seeing was less than or equal to 1 arcsec. Such stable conditions are related to local topography: the mountain has a smooth convex contour facing the prevailing northerly wind and the airflow is comparatively undisturbed. Microthermal tests over a period of six months indicated that seeing conditions are relatively uniform over the whole site. A summary of the results of site testing can be found in an article by Bennet McInnes (Quarterly Journal of the Royal Astronomical Society, vol 22, p266, 1981), and a review of the properties of the site has been published by Paul Murdin (Vistas in Astronomy, vol 28, p449, 1985).
The site testing results have been confirmed by the experience of the first ten years operation of the Isaac Newton Group. In the period 1984-1993, 76 per cent of the dark hours were spectroscopic. Measurements through the INT (which include dome and instrumental effects) indicate that the seeing is subarcsecond for about 50 per cent of the spectroscopic hours: however, no large body of data yet exists of direct measurements of the intrinisic site properties. Occasional measurements of image profiles in very good conditions have yielded typical values of 0.5 - 0.7 arcseconds. The most detailed meteorological records (including extinction) are from the Carlsberg Meridian Circle site. These are available continuously from June 1984 and are published every six months by the Copenhagen University Observatory, the Royal Greenwich Observatory and the Real Instituto y Observatorio de la Armada, San Fernando; they are also stored in the ING information section on the World-Wide Web.
As part of the ``half-arcsecond programme'', a Differential Image Motion Monitor (DIMM) has been installed on a tower near the WHT. A programme of systematic measurments of seeing inside and outside the WHT dome, combined with improved metorological information will be used to search for the most important internal end external influences on seeing. In order to reduce the internal seeing active cooling of the telescope mirrors and of the oil supply to the telescopes will be undertaken.
Table is a summary of clear weather statistics taken from the INT observing log, 1984--1993. Clear weather is expressed as usable (spectroscopic) time as a percentage of astronomical dark hours. Although the number of good nights per month shows a seasonal variation, the greater length of the winter nights means that the number of spectroscopic hours per month is rather uniform throughout the year (see fig ).
Table: Clear weather and seeing statistics
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Figure: Distribution of clear (spectroscopic) hours by month
Typical values of the night sky brightness measured by the JKT are given below. They were measured in good conditions on moonless nights, in magnitudes per square arcsecond. The variation between 1984 and 1989 may be partly due to a correlation with the 11-year Solar cycle.
U: 21.4, B: 22.3, V: 21.4, R: 20.4, I: 19.3 (June 1984)
U: 21.0, B: 21.8, V: 20.8, R: 20.4, I: 19.4 (Sept 1989)
A flux--calibrated, low resolution night sky (airglow) spectrum is shown in Figure .
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Figure: Flux calibrated night sky spectrum, La Palma
Observing conditions for astronomy on La Palma are protected from artificial light pollution by Parliamentary law according to standards that conform to the recommendations of the IAU. However, emission lines of HgI do appear in the sky spectrum, especially if some dust is present.
Measurements of the distribution of extinction in the V band, taken from CAMC observations of 1984--1986, are given in Table .
Table: V band extinction
Extinction coefficients remain essentially constant on clear and dusty nights, and colours ( U-B, B-V, V-R and V-I) are unaffected at the one per cent level. About 2 per cent of the dark hours of the year are rendered unusable because of Sahara dust. There can be temporary anomalies due to volcanic eruptions -- see for example Gemini, 41, p8 and 35, p19 for the effect of the 1991 Mount Pinatubo eruption on La Palma extinction.