TABLE OF CONTENTS
WeatherMean minimum temperatures at the summit are around 0 C (summer) and -4 C (winter). Extreme temperatures hardly ever go lower than -10 C. Daytime temperatures are normally about 10 C in summer and 3 C in winter. Weather conditions in the Hawaiian Islands are determined largely by the strong persistent Northeast Pacific Ocean anticyclone, which usually gives rise to easterly (trade) winds in Hawaii, especially during the summer season. Trade winds give an inversion layer with an average height of 2000m; air above this inversion tends to be both dry and stable, hence giving the good astronomical quality usually experienced at the Observatory. At the mesoscale level, the summit of Mauna Kea is generally intercepting a free flow of air, thus preserving this good quality. However, high altitude cirrus can be a problem; in some years it has been present about 30% of the time. The mean annual precipitation at the summit of Mauna Kea is ~15 cm, most of which falls as snow during the winter.
The median seeing
~0.40 arc sec, with a likely systematic variation between winter (0.45)
summer (0.35). The 10 percentile is probably of the order of 0.25 arc sec. The summit of Mauna Kea appears to be in
that respect, the best known site on earth. Observers must be cautioned, however, that seeing characteristics are often
highly variable, even during the course of a single night.
A large sample of
at prime or F/8 Cassegrain focus, have allowed good statistical study
image quality with CFHT. Images are at the subarcsec level at least 75% of the time and long-exposure images with
FWHM better than 0.4 arcsec have been obtained. The figure below shows the evolution of image quality as
documented by science images taken since the beginning of CFHT operations. Note that the HRCam and SIS images
have been taken with the instruments' fast tip/tilt systems, and that the MOS images are badly under-sampled.
Median image quality
better than 0.8 arc sec. Optical quality of the telescope, dome and
seeing, image motion and guiding errors play a substantial role, and the free atmosphere seeing is usually better as
brightness at zenith
during dark time is given in the table below.
Night sky brightness in
by a factor
of 5 at quarter moon and 65 at full moon. Corresponding values in V
From 1.5 microns to 2.2 microns the spectrum of night sky emission is dominated by OH emission lines; between 2.2m
and 2.55m H2O lines and thermal continuum are the dominant contributors.
Typical spectra, taken from ESO (Chile)
and from UKIRT (Mauna Kea) are included for reference.
in the infrared. Over a few minutes they typically vary by 1% in J, 2%
and 0.3% in K. These figures by T. Gerball, obtained at UKIRT, are highly variable, however, especially for J and H.
Longward of 2.5 m the background emission is set by thermal radiation from the telescope and from the atmosphere.
Mean sky emissivity is 0.35 at 20 m and 0.67 at 27 m.
City lighting is
quite often completely damped from cloud cover at the 2000-3000 m
ordinance has been adopted, which restrict most lights of the Big Island to low-pressure sodium lamps.
an interesting look at our night light
environment at CFHT, see The
Light Environment of Mauna Kea.
Extinction and Refraction
An airmass nomograph for Mauna Kea is given here. Note that a unit airmass at Mauna Kea (with a mean barometric pressure of 605 millibars) is equivalent to 0.60 airmass at sea level.
The time of sunset and sunrise at Mauna Kea throughout the year, and the corresponding sidereal time are provided in the accompanying figures.