Unusual Atmospheric Phenomena by Peter Anderson

We all at one time or another have been struck by the beauty of a sunset. This is an example of an atmospheric phenomena that has the power to enthrall. It can be enhanced by a fine foreground, a clear view, a sea horizon. Turning in the other direction one can then observe in the east, the rising blue purple of the Earth’s shadow projected in the atmosphere. Some call this ‘rising dusk’. However, like a good view from one’s window, these can become familiar and commonplace after a while and it is the rare phenomena that more often catches the imagination.

Into this category are the familiar rainbows and in appropriate conditions, the rarer circles about the sun and moon, often with coloured fringes. The following is a beautiful 'double rainbow' photographed in the late afternoon from our home in the suburb of The Gap. (Brisbane, Australia).

On occasions a well positioned high altitude cloud containing ice crystals, in this case a dense cirrus at 45º from the Sun, will put on a prismatic display as the two images below demonstrate. They were taken a few minutes apart in November 2001. (If this cloud had covered the whole sky, a 22.5º halo with mock suns -see later - and a 45º halo may have been visible.) Naturally ice crystal phenomena are much more common in colder climates.

Then there are the other interesting clouds, but none so weird as the lenticular clouds, often associated with 'flying saucer' reports, here seen in December 2001 over Cape Horn at the tip of South America.

Another interesting cloud phenomenon can be caused by the not yet risen or the set sun shining through clouds that are below the horizon. Seen from a high flying aircraft these clouds would be seen to be casting parallel east-west (morning) or west east (evening) shadows across the lower atmosphere. However these shadows, as seen from the ground underneath, or more particularly the bright areas between the shadows appear as rays that come from the east or west (whether morning or evening), spread out overhead and then converge at the anti solar point. This is particularly spectacular if the atmosphere is a little hazy.

Into this category, and particularly at sunset, is the green flash which is occasionally seen on a flat clear horizon. The effect is caused by refraction of light with the longer red waves being bent less and so seen on the lower edge of the solar disc. The shortest visible waves, violet and blue are bent the most and so appear on the upper limb. However these are usually dissipated into the sky background leaving green remaining. These colour fringes are of insignificant brightness compared to the solar disc, but at the instant the sun slips below the horizon, the ‘green flash’ may be seen if the conditions are right. As the sun sinks lower and we see it through thickening layers of atmosphere the shape of its disc becomes distorted and often reddish , first becoming an oval, and then occasionally a very peculiar flattened shape as it sets. It is the final disappearance of the upper limb we are waiting for.

I had chased the ‘green flash’ phenomenon for years, on a number of cruises from the upper decks of the old ‘Fairstar’ to get the most distant horizon and hence the longest atmospheric path. Other attempts were from mountains, at sunrise over the sea etc. At best the image appeared to pale before it disappeared but there was no green flash. Then in 1985 when spending a western US/ Hawaii holiday with the family we were on the Kona Coast of the big island of Hawaii overlooking a marina with the sun setting in the background. It was the last day of our holiday and we were due to fly back the next morning. A girl on a bicycle came by and casually said that if we waited a few minutes we would see the sun ‘go green’ as it set. Having hunted this phenomena for at least twenty years I was dubious that it could be seen from a mere 10 metres altitude but said nothing. I was so doubtful that I did not even have my camera ready. Then as the sun set there was a flash of brilliant neon green as it disappeared, lasting perhaps half a second. Obviously at the latitude of Hawaii, this phenomena is not unusual.

The next time I saw the green flash well was during an eclipse cruise in the Caribbean in February 1998. There were two good sightings of the green flash during the cruise, but on the first occasion I was still ashore. When word spread among the hundreds of amateur astronomers aboard, they crowded the upper decks hoping for a repeat performance of the green flash. We were amply rewarded. As the upper limb of the sun "winked" below the horizon it turned a brilliant green which was reflected in the water making a green pathway between the setting sun and the ship. The ship erupted in applause. Though solar eclipses themselves produce some spectacular atmospheric effects, they are more the subject for a separate article.

For air travellers a number of interesting observations can be made, including the deep dark blue of the overhead sky at some 10,000 metres altitude. What always fascinates me is the bright subsolar halo which travels with the aeroplane and increases and reduces in size and brightness depending upon the type of surface you are flying over. At this point on the ground directly opposite the sun there are no shadows and so it is very bright. At lower altitudes the small shadow of the plane will be seen in the centre of it. Flying back from Anchorage to Seattle in September 2000, I noticed for the first time a dark bar like a spear which ended at this subsolar point. This was the shadow of our jet contrails which are so prominent at these latitudes.

People living in colder climates are quite familiar with the unusual effects which result from ice crystals in the upper atmosphere. I had my first experience with this one cold December morning in 1989 in Limon, near Denver, Colorado. We had arrived under atrocious conditions the previous night, driving through a snowstorm on icy roads. The morning dawned clear and a little hazy but was minus 27 degrees Centigrade. We literally ran from the motel to the car and drove the 200 metres to our breakfast place, diving inside quickly. In the east, through the large panoramic windows, three suns rose in the sky. After the initial shock, I looked more closely and saw that the mocks suns on either side were 22.5 degrees distant and I knew that they were caused by reflections within ice crystals in the cloud and were associated with a fainter ring around the sun. I have only seen this phenomena once more, in 1999 in Germany. On that occasion the sun was high in the sky and there was a cloud on only one side, and only one mock sun. We are all familiar with rainbows (at times double) and halos around the sun and moon but good mock suns or parhelia take some beating.

July 1999 saw us in Norway heading north as the sun moved south after mid summer. We wanted to see the midnight sun. We were travelling north on the scenic coastal road which went inland to a high valley about 600 metres in altitude where it crossed the arctic circle in an area of desolate tundra. Even in mid summer it was cold and rainy, almost to the point of snowing. In our quest to see the midnight sun we pushed on past Narvik, and then decided to try our luck from the spectacular Lofoten Islands. The rain persisted so we abandoned this plan and continued north. We knew that the sun was travelling south as we were travelling north and there were only a few days left before we ran out of Norway! In an endeavour to get a clear northern horizon in this mountainous landscape, we turned off to Tromso, a northern university town, and after some difficulty found a spot on a peninsula with a view to the north. It is difficult to find a place with a clear view to the north because the bad weather comes from this direction so the towns and roads are in the more sheltered fjords. At midnight the sky was totally cloudy but cleared by breakfast time. We were somewhat heartened by the clear weather and continued on our race north again. Passing 70 degrees latitude north, we came to the town of Alta and snuck our motorhome in behind the local Statoil Service Station. From there we had a clear view over a wide expanse of water down a fjord with low hills in the distance to the north. The sky was clear apart from a few wispy clouds. Over the next six hours the sun slowly curved down to the horizon, scraped its lower limb along the hills and continued on its way again. These hills were wonderful reference points. Allowing for various factors local midnight had been around 12.30am. I took photos at regular intervals, winding the window down and up quickly to beat the huge mosquitoes. From time to time people almost reverently walked out to where we were and took photos of each other in the midnight sun, but never of the sun itself. By 1.45am the sun had arced up enough for the heat to return, and the curious humid arctic summer of around 30degrees Centigrade continued. In all by the time we went south out of the area, the sun had not set for nearly a week and it had not been totally dark the entire time we were in Scandinavia. We welcomed the dark of night when it finally came. Something primal in our bodies told us that this constant light was unnatural.

At high latitudes such as this the sun, moon and planets conspire to confuse directions. A good example is the moon which, with its 5 degree extra inclination to the ecliptic can travel more than 28 degrees north and south declination. As a consequence at 62 degrees north, and well below the arctic circle it can either remain in the sky all of the time, or not be seen at all. Furthermore because of the geometry it does not necessarily rise in the east, nor set in the west. My friend in Alaska put it simply. He said that it could rise nearly anywhere in the eastern hemisphere and set nearly anywhere in the western and in between its altitude was generally so low and the resulting image so poor from bad seeing, that he wondered whether his telescope was functioning properly. For my part, I was impressed with the way the crescent moon stood and set ‘upright’ on it’s pointy end, as well as the usual ‘reversed’ northern hemisphere sky.

With solar maximum occurring in 2000, we decided to try to see the Aurora Borealis. We considered flying into Tromso in Norway until we decided to combine the quest to see the aurora with a trip to Alaska – somewhere we had not been. We had both seen the Aurora Australis from Brisbane, but only as a pink diffuse glow in the southern sky on two occasions in 1957. (I had heard of several local reports in the late 80’s, one of faint auroral curtains on the southern horizon after a lunar graze excursion, but I had not seen anything myself.) We planned a fairly long trip which started with the cruise northwards on the Inside Passage from Vancouver and ending at Seward near Anchorage. We would then travel for 11 days around southern Alaska by motorhome which was timed to give us a good chance of seeing the Aurora. A fair bit of research went into this beforehand with shipping schedules, sunrise/set times, the sun’s negative elevation beneath the horizon, absence of moon, with all factors having to be just right in the period between mid August and around the end of the first week in September when Alaska shuts down for the winter. The zone on the earth with the most spectacular auroral displays is around 65 degrees north, the latitude of Fairbanks, and I was determined to spend several nights in that region.

The north west coast of Canada and southern Alaska are renowned for a number of things, one of them being rain. This ran largely true to form but we did have one clear night in the Gulf of Alaska at 61 degrees north before disembarking at Seward. Midnight on 22/23 August was our first unsuccessful attempt to see the aurora and I was trying to shield myself from the ship’s lights and spy out the constellations. Then my befuddled mind realised that the glow from sunset that I thought was in the western area was actually in the NORTH because the sun was not far below the horizon. This is weird – the sunset glow in the north! As soon as I realised this, I had my bearings and the star patterns crystallised. Above the glow, there it was, the big dipper, Ursa Major spread out under the pole which was 61 degrees high in the sky with Lyra and Cygnus virtually overhead.

After we left the ship and collected our motorhome in Anchorage, we set off northwards, sightseeing, wildlife spotting and on a quest to see the aurora. (You can find the images of the aurora in the article in the 'Observations' section.)

A high latitude ring of more or less permanent aurora circles the polar regions at around 65degrees latitude. Effectively in Alaska, this is directly over Fairbanks where we were heading. The aurora is the emission of light from Earth’s upper atmosphere. It is caused principally by Oxygen atoms or Nitrogen molecules which are excited by electrons accelerated within the magnetosphere. Depending upon the atom or molecule concerned and the level of excitation the colour of the aurora can range from purple to red, but it is predominately green or a greenish white. Naturally the activity and intensity of aurora correlate to the 11 year solar cycle since it is related to the emissions of charged particles by the sun.

The first few nights ashore were disappointing with overcast and light rain. Cloud and drizzle persisted and in Denali National Park it snowed in the higher areas. The weather is incredibly changeable and it cleared by the time we arrived in Fairbanks. We set up on a little used dirt road and waited, but the first night was disappointing. Apart from the strange sunset glow in the north - under the circumpolar big dipper, there was only a faint cloudlike auroral band from the eastern horizon stretching across the sky towards the west. This is presumably part of the band that is always present to a greater or lesser extent circling the magnetic poles at around 65 degrees latitude, and we were directly under it. After a day around Fairbanks, including time at the Fairbanks Museum which has incredible auroral exhibits and videos, we tried again the second night. It was fine when we went to bed but, by midnight it was overcast apart from a few degrees of clear sky on the eastern horizon. On that eastern horizon though, there wavered pillars of auroral light which lit up the overcast from above. From time to time the clouds would illuminate with streaks of light. The pulsing of the aurora gave an appearance of lights dancing through thinner areas of cloud. One I remember was a huge ‘Z’ like the mark of Zorro however we could only see these filtered through the overcast. It was terribly frustrating because we knew there was a great auroral display going on, but we could not see it.

The next day we moved on and night found us at remote Fielding Lake campsite, well away from any lights. Come midnight we were outside watching a magnificent auroral display, but what was that splashing in the stream about 20 metres away? Was it a moose or a bear? Was it coming closer? Though we stayed near the motorhome door, not all of us could fit through it at the same time. The aurora however proved too alluring as it danced, wavered, pulsed, and flickered, covering nearly all of the sky except the far south. Still photos cannot do it justice and a low light video I had seen made it seem somewhat detached from the personal experience it presented.

Imagine the predominate band from east to west, slowly moving, but at times accompanied by other bands, generally in the north. The overhead aurora as seen from below radiated out in a splash. Areas of the phenomena pulsed brighter and then faded to near invisibility every second or two, but then could flicker on and off four or five times per second. The light from the aurora was so bright that you could read a large print book. Though there was some slow movement against the sky, the familiar curtains which formed, dissipated, and reformed did not move as curtains do in a breeze, but areas flickered on and off which gave a strong impression of motion. The display continued for perhaps two hours and then died down. The colour was predominantly white or light green. Totally frozen, we crept off to bed and discovered in the morning that it had been a moose nearby during the night. His tracks were near the van and he and two friends were in the stream running past the motorhome. Though not as dangerous as a bear, a moose is quite likely to charge on sight and so must be kept at a safe distance.

Over the following nights that were cloud free there were muted displays which I keep track of out of the window at the head of my bed. It was easy to check a few times during the night to see if there was an aurora visible. There was usually a faint auroral band crossing the sky from east to west and some non specific auroral glows towards the north. These auroral glows to the north are quite common, in fact ‘aurora borealis’ translates as ‘northern dawn’. I began to appreciate the comments by my Alaskan amateur astronomer friend that the sky never truly gets dark because there is always some auroral activity. I found it hard even in very dark sites, to look through this auroral haze at stars and star clusters etc.

Our next exceptional night was at the Clam Gulch State Park on the Kenai Peninsula. A look out the window near midnight revealed the aurora dancing. Rugged up in parkas we piled out to look at a most magnificent all-sky display that went on, and on, and on. We had to keep swivelling our heads around so we didn’t miss some of the spectacular bits. At times the auroral curtain was directly overhead which looked incredible. Fatigue and cold sent the others back to bed after a couple of hours but I stayed until the brightening dawn sky caused the aurora to pale.

Though I looked hopefully at the sky near midnight when we were in the Canadian Rockies and the northern US, we did not see the aurora again. The images of the aurora will remain one of our lasting memories of our trip to Alaska.