About noctilucent clouds
Noctilucent clouds visible in the faint twilight arch that occurs
throughout the summer nights in the Netherlands. Note the difference in appearance
from tropospheric clouds which appear dark in front of the arch.
Noctilucent clouds are called such because they appear to shine
during the night. Since they are much higher than ordinary clouds
(around 82 km altitude as opposed to below about 12 km altitude for
common tropospheric clouds) they
can still be illuminated by the sun if it is low under the horizon.
Noctilucent clouds can only exist during the summer months (May
through August on the northern hemisphere) because this is the time
(paradoxically) that the air is coldest at that altitude. It is
thought that they form due to sublimation of the little water vapor
in the mesosphere onto dust particles (perhaps meteoritic) that act
as freezing nuclei.
Observing noctilucent clouds
Noctilucent clouds are only visible from latitudes between around
50o to 60o north or south. If you are too
far in the arctic during the summer months, the sun will either not
set or be so close to the horizon that it won't be quite dark enough.
Further towards the equator the clouds do not occur and the clouds that
do occur will be below the horizon for you.
Noctilucent clouds cannot be forecast, so if you want to observe
them you will have to check the sky from time to time during
every clear night. They occur typically around latitudes of
60o north or south, so depending on your latitude you
may have to look north, south or overhead. Once you see them, they are
After the sun has set and civil twilight has ended (after about
30 to 45 minutes after sunset), scan the sky for silvery white
clouds. They will appear different from tropospheric clouds such
as cirrus. Cirrus and other clouds will appear dark in front of
the twilight arch, while noctilucent clouds will appear to shine
and be brighter. Also, using binoculars you will see more detail
in noctilucent clouds while using binoculars on cirrus will show
little more detail.
Various forms of noctilucent clouds
Noctilucent clouds can appear as a structureless veil, bands, waves,
undulations, a net-like structure, whirls, or other complex forms.
Bands, waves and whirls are most likely. All shapes (except maybe
the veil) are spectacular to watch. To learn more about how to
observe these clouds and which forms are possible, visit the appropriate
section at the NLC observers' site.
Noctilucent clouds, if they occur, become visible during
nautical twilight, about one hour after sunset. Usually they are
Photographing noctilucent clouds using film
For photography, you need fast film, and relatively short exposure
times, since these clouds move very fast. Try to keep exposure
times under 10 seconds for a standard or telephoto lens, and under
20 seconds for wide-angle lenses. I suggest using 400 or 800 ISO
film, a shutter speed of 10 seconds, and f/ratio of 2.8 or 4.
It is very important to bracket your exposures, and use lenses with
various focal lengths, to get a variety of photos. Keep in mind
that wide-angle lenses will show vignetting at large apertures,
so you may want to increase exposure time somewhat and reduce the
aperture (choose a higher f/ratio number).
Photographing noctilucent clouds digitally
If you have a digital SLR camera, I suggest using this instead of
film, since you can see the results immediately and decide if the
exposure time is good. Noctilucent clouds can vary much in
brightness and the exposure is somewhat critical. I recommend
using one of Canon's newer DSLRs such as the 300D, 10D or 20D, or
Nikon's D70. Use 400 or 800 ISO sensitivity.
I suggest taking a darkframe for every shutter speed and ISO
setting and do darkframe subtraction later in computer rather than
using the camera's noise reduction mode in the field. This will
save you some valuable time (noctilucent clouds may not last very
long). But most likely there won't be much hot-pixel noise anyway
for exposures of a few seconds.
For all photography, by all means use a tripod and cable release.
Undulations, whirls and net-structure are great to zoom in to with
a telephoto lens. Keep exposure times short to not have the photo
blurred. For a 200mm telephoto lens, do not expose longer than 5
to 10 seconds.
Some noctilucent clouds will be bright enough to cast shadows, and
be bright enough for your camera light meter to function. If it
does and you are using film, you may want to overexpose one stop
to compensate for reciprocity errors.
Noctilucent clouds and the sky around them are slightly polarized,
but in the same direction, so polarizers should have little effect
except to darken the entire frame a few stops, which is undesirable due
to the low light. I never tried using a polarizer, but you can experiment
Noctilucent clouds often move and evolve extremely fast. I
recommend you to take a series of exposures at regularly spaced intervals
or use a low-light capable video camera to do a timelapse.