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ski slope lighting



Dear Nancy Clanton,

David Crawford recommended me to ask you, when I looked around (at the
European DarkSky symposium in Lucerne) to find somebody who has an
experience with lighting of slopes for downhill skiing. He was sure you
have.

A National Park in northern Czechia demanded an Environmental Impact
Assessment concerning a project to light one slope there, and I am in fact
the only Czech expert in environmental consequences of outdoor lighting.

A colleague from Catalonia said me that ski lighting will be forbidden
there. This is of course the most environment-friendly possibility.
However, it would demand strong arguments and might be very unpopular in
Czechia -- there are some lit slopes already, even if not in national
parks. I'd prefer some compromise solution to show, that light pollution
prevention does not imply an end to the nowadays way of life, but rather
brings a new quality to it.

Therefore my draft recommendation has been to use decent amount of light,
giving the snow the luminance around the top limit of the values
recommended for roads, namely not exceeding 1 cd/m2. Even such a luminance
of a long slope would illuminate substantially a vast area on the opposite
side of the valley.  Some five lux of horizontal illuminance should give
that 1 cd/m2 perhaps, even with the least white snow. The skiers
themselves would be illuminated more on the average, thanks to the light
from the snow; even in places with no direct vertical illumination they
should be lit by some two lux. I suppose that such values should do, if
the light from the luminaires above 75 degrees from the slope will be kept
to minimum (i.e., with more than FCO demands).

The project mentioned however an average horizontal illuminance of 20 lux
for each evening, with a possibility of 100 lux for occasional
competitions. 20 lux is mentioned in some technical standard for skiing.
If I compare it with the recommended 4 lux for areas with many pedestrians
and cyclists (cyclists are going more often in opposite directions),
it seems to be an overkill to me. The slope has an only simple lift, so
the traffic on the slope will be kept low.

Another argument for not so large light levels is, that the surrounding
paths won't be lit -- the lift itself (being divided from the downhill
slope by a strip of forest) and the paths (some are in a dense forest) to
the other parts of that skiing resort. With too much light on the slope, a
proper adaptation to move safely on those paths would demand too long time
to happen in reality. In view of this fact, perhaps 2 lux horiz. illum. on
the slope would be better than five.

The owner argues however that some previous experience showed, that even
20 lx is not enough, that there have been some collisions. My answer was
they have been quite possibly due to glare. The more dangerous as the
skiers can carry glasses with snow or drops on them. Low glare should be
more productive than lots of light. 

I'd like to adhere to the 1 cd/m2 limit, as we plan to put it into the
Czech regulations, for all cases where safety standards don't demand more.

I would not say that 20 lx for skiing is a safety standard. If it would
be, than the skiing on slopes with no artificial lighting should be
abandoned even before sunset on the most heavily overcast days (say, with
just some man-made snow on the slope and a dark landscape elsewhere), but
all lifts operate till sunset in December and January. The more because
the omnidirectional light from an overcast sky hinders recognition of any
shapes on the slope, unlike sunlight or light from luminaires.

But, your experience and opinion would be most authoritative for me. If
you would agree with the low light levels, than that skiing slope could
become a landmark in sustainable sports lighting. Otherwise I would rather
recommend to avoid such a disturbance amidst a national park (even if
there is a town in its centre). 

sorry for a long letter,
 your sincerely 
 Jenik Hollan, IDA Czech Republic