Atmospheric optical phenomena An introductory guide by Mike Feist Effects caused by water droplets— rainbows and coronae The most well known optical sky effect is the rainbow. This, as most people know, sometimes occurs when the Sun is out and it is raining. To see a rainbow you must stand with your back to the Sun with the raindrops in front of you. It does not have to be raining where you are standing but in the direction that you are looking. The arc of the primary (main) bow is centred on the antisolar point, the spot directly oppo- site the Sun, and has a radius of 42°. The antisolar point is actually centred on the shadow of your head. If the Sun is rising or setting and therefore on the horizon, the primary rainbow will be a complete semi- circle and the top will be 42° up in the sky. If, on the other hand, the Sun is 42° up in the sky, the primary bow will be on the horizon, the top just rising or setting. Con- ventionally the rainbow is said to have John Constable. Hampstead Heath with a Rainbow (1836). seven colours but all we need to remember seen in the spray near waterfalls and artifi- ous forms but with a six-sided shape. They is that, in the primary bow, the red is on cial rainbows can be made using a garden may be as flat hexagonal plates or long the outside and the blue on the inside. Out- hose. Rainbows are one of the easiest opti- hexagonal prisms or as a combination of side the primary bow sometimes there is cal effects to photograph although they the two. Such crystals have 120° internal fainter bow with a radius of 51° and in that may not completely fit into the field of angles with 90° angles on the ends. colours are reversed. Between the two, the view of your camera. Sunlight can either be refracted through sky looks much darker. This area is known Water droplets in the clouds can also these crystals as it would through a stan- as Alexander's Dark Band. Within the pri- produce a corona. This appears as a small- dard prism, (a hexagonal prism can be mary bow other faint bands can sometimes ish ring, or rings, around the Sun and imagined to be a 60° triangular prism with be seen. These are called supernumerary Moon when they are shining through the angles removed.) or be reflected off bows and are faintly coloured. clouds. They are often outlined with a their surfaces to produce either colourful The brightness and richness of the col- brownish red colour and are usually about or white arcs or patches of light. The ori- ours of the rainbow depends on the size of 5° or less across and should not be con- entation of the crystals is important, as is the water droplets producing them. Fog, fused with the solar corona, an outer part whether they are spinning or not. Such ice which is made up of very small droplets, of the Sun, visible during a solar eclipse. crystals mainly occur in wispy cirrus can produce a fogbow, which is colourless. The atmospheric corona is close to the Sun clouds or sheets of thin cirrostratus cloud The Moon can also produce a rainbow, in the sky it is therefore hard, or even dan- and even aeroplane condensation trails known as a moonbow. This is necessarily gerous, to look at and most observed coro- (contrails) may produce colourful and bril- quite faint, the colours not being notice- nae will be around the Moon. liant effects. Both the Sun and Moon can able to our eyes. Reflection and refraction Effects caused by ice crystals—patches, cause such effects although only the within the raindrops cause nearly all these arcs and circles brighter forms can be seen around the effects, although the supernumeraries are Water in high clouds is often frozen into Moon because our satellite is faint in com- caused by diffraction. Rainbows may be ice crystals. These crystals occur in vari- parison to the Sun. The commonest phenomenon is the 22° halo that can be seen around either the Sun or Moon. Many observers must have no- ticed this huge arc, like a great cartwheel in the sky, around a rather murky Moon. It is common in the daytime around the Sun although this is less likely to be noticed due to the glare. To protect our eyes al- ways hide the Sun behind your hand or roof or street-lamp (or something similar) when observing haloes. Not all the halo may be visible, as it will only occur where the cloud is and the cloud may not com- pletely cover the sky. The bottom half of the halo may be too near the horizon to be clearly seen and the commonest effect re- corded is the 'upper part of a 22° halo'. Lunar haloes are less common as the Moon is not in the sky every night and, when it is, it will often be of a small phase which is not bright enough to produce a halo. However, the Moon does not have to be Full to produce a halo, even Quarter Moons will do it. The Solar 22° halo may be fairly colourless although often will This magnificent solar halo was imaged by SPA Solar Section Director Richard Bailey. show a reddish edge. It will never be as Is it raining? Is it centred on the light source, the zenith or antisolar point? Take care not to look directly at the Sun but block it out using a book or building. Is it a patch, an arc or a circle? An arc may be an incomplete circle. How many degrees is it from the light source? Use your outspread hand at arm's length as 22° measure. Is it coloured or white? Look carefully as the colour may be very faint. Are there any other optical effects visible? If one occurs, there may well be others in another part of the sky. What types of clouds are there in the sky? Cirrus and cirrostratus—even contrails— tend to produce good effects. Some angular sizes found in atmos- pheric optical displays ½° Diameter of Sun & Moon 2-4° Diameter of lunar corona 22° Radius of common (small) halo 42° Radius of primary rainbow 46° Radius of (large) halo 51° Radius of secondary rainbow Percentage frequency of atmospheric phenomena (based on observations made from January to April 1990) Solar halo 49% A sun pillar, imaged by Graham Thornton in Portsmouth. Parhelia 25% colourful as a rainbow, although there are tion, it will never show coloured effects. It Rainbow 10% other phenomena that can be. A halo oc- is know as a parhelic circle. Lunar halo 4% curs in the direction of the Sun (or Moon) Another phenomenon produced solely Lunar corona 4% but a rainbow (or moonbow) must be op- by reflection is the Sun pillar (or Moon Sun pillar 4% posite the source, so that they can easily be pillar). When the Sun approaches the hori- Circumzenithal arc 2% told apart. zon at sunset, a single beam of light may Unidentified 2% Rarely, an even larger halo will appear sometimes be seen shooting straight up around the Sun. This is called the 46° solar from it. More rarely a beam may be seen Bibliography halo. As it is much fainter than the smaller reflected below it. Similar effects may be Colour and Light in Nature halo and is so big, over 90° across, it is seen at sunrise. Such effects must not be ByLynch & Livingstone (2001) CUP almost never seen in its entirety. Some- confused with crepuscular rays which ap- ISBN 0521775043 times haloes with other radii are seen and pear as radiating beams, above or below How to Identify–Weather these are often hard to explain. the Sun, when it sunlight shines through By Storm Dunlop (2002) HarperCollins The most colourful effect must be the gaps in the clouds. Recently a Venus pillar ISBN 00022026 circumzenithal arc, which appears above has been recorded and even photographed. Out of the Blue the top of the 46° halo. It is unusual in I have left to the end one of the most By John Taylor (2002) CUP that, unlike many haloes, it is not centred common optical phenomena, which has ISBN 0521809258 on the Sun but on the point vertically three names, It is scientifically called a Rainbows, Halos & Glories above, called the zenith. It looks like, and parhelion (plural parhelia) but is also By Robert Greenler (1989) CUP is often misidentified as an upside down known as a mock sun or sundog. The lunar ISBN 0521388651 rainbow. The Sun has to be quite low in version is called a paraselene or moondog. Seeing the Sky the sky for it to appear. Sundogs occur about 22° either side of the By Fred Schaaf (1990) John Wiley ISBN Below the bottom of the 46° halo there setting or rising Sun but will be farther out 047151067X might appear a circumhorizontal arc. This when the Sun is higher in the sky. If the The Nature of Light & Colour in the Open is also very colourful but the Sun has to be Sun is higher than about 60° the sundogs Air very high in the sky for it to appear at all cannot form at all. Sundogs often occur on By M/Minneart (1954) Dover and is unlikely to be seen from out lati- their own without the halo being present ISBN 486201961 tude, although it is just a rare possibility at and they may occur singly or with one on Wonders of the Sky the very height of summer.