Noctilucent Cloud Observation Report

Forrest M. Mims III

A small but distinctive noctilucent cloud (NLC) display (Fig. 1) was observed after midnight on Friday morning (28 July 2006) from just above the parking area at Flat Top Mountain near Anchorage, Alaska. Observers included 17 participants and 4 staff of the AIM/GLOBE/CloudSat 2006 Workshop. Three additional students observed the NLC display from the campus of the University of Alaska. This event climaxed a field trip that began at 5:00 AM the previous morning and was the highlight of a workshop devoted to this potentially significant but rare atmospheric phenomenon.

Figure 1. Noctilucent clouds over Anchorage, Alaska, 28 July 2006, 01:52 local time. Tripod-mounted Canon EOS 30D SLR at 1 sec., f/4, 40 mm, ISO 100. The image has been cropped and reduced to a width of 800 pixels. Minor contrast enhancement was applied. Photograph by Forrest M. Mims III.

A slightly earlier view of the NLC display looking more to the east is shown in Fig. 2. The image has been enhanced during post-processing to better reveal the NLCs.

Figure 2. Noctilucent clouds over Anchorage Alaska, 28 July 2006, 01:44 local time. Tripod-mounted Nikon D70s SLR at 20 sec, f/22, 110 mm, ISO 200. This image has been enhanced to emphasize the NLCs. Photograph by Matt Rogers, Colorado State University. Copyright 2006 by Matt Rogers.

Most of the observers photographed the NLC display using Panasonic Lumix cameras mounted on tripods provided to them by the workshop. Sample photographs made with these cameras are given below.

Highlights of the NLC observations included the following:

1. NLC PRECURSORS. The NLCs were visually detected by an experienced twilight observer before the clouds became obvious. During this phase, the clouds appeared as subtle, light features above the greenish layer of the twilight glow. This finding was tentatively verified by enhancing digital images.

2. NLC DYNAMICS. Obvious changes in the structure of a braided NLC occurred over a few minutes. This suggests that animated GIFs and elapsed time videos can provide a useful means for analyzing NLC dynamics.

3. NLC STRUCTURE. The primary NLC that was observed exhibited braiding and, for a time, two distinctive, parallel bands were apparent. Rather than two distinct layers, it is possible that these “layers” might have been the segments of a broken NLC viewed edge-on. However, in view of distinct, closely-spaced, parallel layers that often occur within the ozone layer and the troposphere, layers within the NLC region cannot be ruled out with certainty based on this single observation.

4. NLCs vs. CIRRUS. The NLC display occurred visually adjacent to cirrus clouds. This provided an excellent opportunity to photograph bright NLCs near dark cirrus. The lesson was obvious to all the observers, who will now be able to easily tell the difference between cirrus and NLCs.

5. CAMERA SETTINGS. The optimum camera settings were discovered by trial and error. The 1-second maximum exposure of the Lumix camera was found to be acceptable. (Much longer exposures were made with SLRs, some of which were overexposed.) Even photos made at the minimum ISO setting (80) captured NLCs. The Lumix imagery is much nosier than images made with higher quality cameras, and application of a noise reduction algorithm could be helpful.

6. POST PROCESSING. Post-processing of digital images can exaggerate the features of an NLC to reveal subtle structures. (Such processing may redden twilight coloration and should be explained when enhanced images are published or displayed during presentations.)

7. RED LIGHT SOURCE. A red LED head-mounted light is ideal for making camera adjustments without interfering with dark-adapted vision.

8. TRIPOD. A tripod is essential, and use of the camera's timer is important. Examples of the effect of camera shake (even with a tripod) were shown to the participants in class. This problem can be avoided by using the camera's timer function so that the camera is untouched during the exposure. An alternative to a tripod is to place the camera on a rigid support. A Casio 850 (8 megapixels with manually adjustable shutter speed) pressed against a wood fence provided very stable imagery. (An interesting exception to the rule is that one participant with a very steady hand acquired a handheld ½ second image of the NLC display from a car that stopped along a road leading away from Flat Top. See Fig. 6 and related discussion below.)

9. OBSERVER FATIGUE. The viewing opportunity occurred at the end of a day that began for most at 4:00 AM. When possible, NLC observation sessions should be scheduled so that observers are well rested and able to stay up through the entire viewing period.

10. NLC PHOTOGRAPHS WITH A POINT-AND-SHOOT DIGITAL CAMERA.

Teachers provided with Panasonic Lumix DMC-LZ2 digital cameras and tripods managed to acquire photographs of the NLC display even though they had minimal experience with the cameras. Several examples are given here. While the Lumix imagery is much nosier than images acquired with the SLR cameras in Figs 1 and 2, the price of the Lumix camera was less than 10 percent of the price of an SLR camera with a suitable lens. An unexpected benefit of some 20 teachers photographing the clouds was the wide variety of camera setting that were employed. These exposure settings will be very useful in planning future NLC photography.

Carolyn Sior acquired the image of the NLC display in Fig. 3. The main portion of the NLC display is above the tree line marching up the side of the slope at right. The dark objects are cirrus clouds in the shadow of the Earth.

Figure 3. Image of NLC display acquired by a tripod-mounted Panasonic Lumix DMC-LZ2 digital camera on 28 July 2006 at 2 sec., f/3.2, 11 mm, ISO 80. The image has been cropped and reduced to a width of 800 pixels. No noise reduction or other post-processing has been applied. Photograph by Carolyn Sior.

Cindy Knisely made the photo of the NLC display in Fig. 4. The NLCs are most noticeable over the bright portion of the twilight glow just left of center.

Figure 4. Image of NLC display acquired by a tripod-mounted Panasonic Lumix DMC-LZ2 digital camera on 28 July 2006 at 1/8 sec., f/2.5, 6 mm, ISO 400. The image has been cropped and reduced to a width of 800 pixels. No noise reduction or other post-processing has been applied. Photograph by Cindy Knisely.

Marsha Willis took the zoomed-in image of the NLC display in Fig. 5. The NLCs in this and Figs. 3-5 can be made more obvious by means of post processing to remove noise and enhance contrast.

Figure 5. Image of NLC display acquired by a tripod-mounted Panasonic Lumix DMC-LZ2 digital camera on 28 July 2006 at 1/2 sec., f/4.5, 37 mm, ISO 100. The image has been cropped and reduced to a width of 800 pixels. No noise reduction or other post-processing has been applied. Photograph by Marsha Willis.

Figure 6 is among the most interesting images of the NLC display because of the circumstances under which it was made. This photograph was made by Wayne R. Gilchrest, Teacher of Marine & Environmental Science at the Chesapeake Bay Governor's School in Bowling Green, Virginia. When he and others in his car noticed that the NLC display had brightened somewhat as they were driving along a road away from the observing site, they stopped their car and Gilchrest successfully acquired the handheld 1/2-second handheld photograph of the NLC display shown in Fig. 6.

Figure 6. Image of NLC display acquired by a handheld Panasonic Lumix DMC-LZ2 digital camera on 28 July 2006 at 1/2 sec., f/2.8, 6 mm, ISO 400. The image has been cropped and reduced to a width of 800 pixels. No noise reduction or other post-processing has been applied. Photograph by Wayne R. Gilchrest.

Wayne Gilchrest described this experience as follows: “While leaving Flat Top Mountain after observing noctilucent clouds (NLCs), the cloud formation became more visible (01:56 ADT, 28 July 2006). Taken while hanging out of the passenger side rear window of the stopped car, supported by the seatbelt, the Panasonic DMC-LZ2 camera was set to "night scenery" mode (f/2.8, ISO 400, 1/2 sec.). Holding the camera in a modified asymmetrical pistol grip, the photographer activated the two second shutter delay with his thumb and released the shutter for 1/2 sec. Two exposures were taken, one with fill flash to reveal the car and one without the flash.”

Wayne Gilchrest has a steady hand indeed, perhaps due in part to his former position as a National Park Ranger responsible for law enforcement and resource management.

The author received a consulting fee and travel reimbursement for participation in the AIM workshop. This article was not solicited by the AIM program or any of its participants and was prepared for The Citizen Scientist under the auspices of the Society for Amateur Scientists.