WHT time lost to bad weather
On average, 27% of observing time is lost to bad weather.
The fraction varies between 15 and 35% from year to year.
Clear nights are almost guaranteed in
June and July, while in December, the chance of clear sky is
only 60%.
Saharan dust significantly raises the extinction (by up to 1 mag) during
July, August, September, and in recent years, in the early part of
the year as well.
At other good observing sites, typically
25 - 35% of observing time is lost to bad weather.
The plot below
shows for each month the fraction of night between dusk and dawn astronomical
twilights when the planned observations could not be carried out due
to bad weather. Engineering/commissioning nights,
service/discretionary nights and nights when observers were using their
own instrument, are excluded from the data.
The raw data (more up-to-date) from which the plot was constructed are
here.
At the WHT, the dome is closed (i.e. time is lost to 'bad weather')
if any of the following apply:
- The sky is clouded over
- The relative humidity exceeds 90%
(after which the domes are not re-opened until the humidity has
stayed below 80% for ~ 15 mins)
- Ice prevents safe opening or rotation of the dome
- The wind exceeds 80 km/h (gust)
- There is enough dust in the air that the beam from a torch is clearly
visible
1998 Feb 19, last revised 2009 Mar 23
