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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 often unmistakable.

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 very inconspicuous.

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.

Some notes

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 with it.

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.