March 5, 2010
Over recent days there have been a variety of transient atmospheric phenomena, fleeting lights in the daytime sky. They are mostly associated with thin high cloud, Cirrus, Cirrocumulus and Cirrostratus and their various species – yes, clouds have species! As these clouds are mostly composed of minute ice crystals when the sun shines through them, depending on the shape and angle of those crystals, they may act as tiny prisms refracting the light and splitting them into rainbow colours.
To see them, first you have to be aware that conditions are right for them to occur. Sunshine is a precondition, plus any amount of the aforementioned cloud to make it hazy. Then you have to look for them. Unlike ordinary rainbows which occur opposite the sun, most of these other effects occur in the direction of the sun. We rarely look that way, with good reason – it can permanently damage your eyes. So wear good sunglasses, position yourself with something between you to screen the sun, such as a tree, or shield it with a hand or a book.
The commonest phenomenon is the 22° halo. An outstretched hand at arm’s length should cover the sun with the thumb, while the tip of the little finger rests on the circumference of the ring. This won’t be enough to shield your eyes though. For most of this day’s observing I stood in the shadow of our chimney stack. The 22° halo sometimes has additions – extra rings or arcs associated with it, some of which are very rare, and for that reason plus its sheer beauty it is always worth photographing, in my opinion.
Commonly associated with halos are sundogs – there is one faintly visible in the halo photo and a second taken at sunrise on the following day.
Apparently, these are formed by light refracting through crystals that are shaped differently from those that create halos. The two together tells of a mix of crystals in the high atmosphere and also extends the possibilities of what light shows may be seen. Sundogs sometimes have a white outer extension. The right hand sundog from the sunrise display began to stretch some distance and was very intense.
This was the beginning of a parhelic circle (par – through, helic from Greek helios – sun). Occasionally a white line may encircle the whole sky, passing through the sun. I have observed this only twice and it is quite a surprising sight, I can assure you!
The final crystal induced effect was a circumzenithal arc (circum – around, zenith – sun’s highest point in the sky). It happened when the sun was low and occurs quite a distance from the sun. It resembles an upside down rainbow with the centre of the bow sunwards and red on the lower surface. Although the colours can be intense it is difficult to photograph, in my experience.
The particular conditions that brought about these effects – a collision between cool and warm air masses – also created a different optical phenomenon – irisation or iridescent cloud. Newly condensing cloud, also at fairly high altitude, may be formed of super-cooled water droplets rather than ice crystals. These refract light differently and produce beautiful pastel hues close to the sun – sometimes too close for comfort. I was lucky with the position of the chimney stack and I mostly let my camera do the looking. Wearing sunglasses will allow you to observe this effect more often. The same thing occurred next morning but the light was too intense to photograph though I saw it clearly through my shades.
What does all this mean to bushcrafters? Well, it is always good to improve your powers of observation and your awareness of Nature. On the day that most of these phenomena occurred, it was evident to me from watching, that the clouds aloft were moving in the opposite direction from the wind and clouds near ground level. The upper air was moving in from the SW and therefore likely to be warm and moist originally. The lower wind was a chilly ENE, a cold air mass undercutting the warm and forcing it upwards where it cooled, showing an incredible mix of lively Cirrus formations and very long aircraft condensation trails, indicating that it was unstable. Something similar can be created by the approach of a warm front, but in this case the nearest front was halted over Ireland so the weather did not deteriorate further. It was worth keeping an eye on all the same. The atmospheric phenomena I witnessed can portend a change and with the warming of the Northern Hemisphere as Spring approaches there will be further collisions between warm and cold air masses over the British Isles which will bring about further opportunities to observe lights in the sky.
Although halos and sundogs have been known of for thousands of years (Pliny the Elder wrote about them in his Natural History from the 1st Century) a number of explorers and observers of Nature, notably William Edward Parry in 1820, Tobias Lowitz in 1790 and Gerald E Owen in 1935 were the first to see and record the rarer arcs and related phenomena. Some managed to have arcs named after them. While today it is a subject of proper scientific study the amateur observer still has a role to play.