Phenomena of the lower atmosphere

The presence of water droplets, ice crystals and dust particles in the atmosphere and local abnormalities in the distribution of temperature and humidity give rise to a wide variety of interesting and beautiful phenomena which are mainly due to either the reflection, refraction or scattering of the rays of the sun or moon. Many of these 'optical phenomena' are described below. Also included are descriptions of a few phenomena which are due to other causes and which are equally worthy of careful observation when they occur.

A mirage is produced by refraction and/or reflection of light in the layers of air close to the Earth's surface. Two main classes of mirage occur, (a) inferior and (b) superior, in which the virtual image is below and above the object, respectively. The inferior image is seen over a flat, strongly heated surface (e.g. desert) and gives the illusion of an expanse of water if it reflects the sky. A superior mirage is seen in the sky at low altitude where a temperature inversion forms a discontinuity between air at different temperatures. Light from an object is, in – this case, reflected off the discontinuity. Such mirages can also show multiple images, the upright ones being caused by refraction through discontinuity. Mirages can also involve magnification (in one or two directions).

Mention should be made of the Novaya Zemlya mirage, a phenomenon in which an astronomical body is seen to rise or set when it is in fact well below the horizon. It can be caused by sighting across a cold ocean or ice expanse, where the surface is much colder than the air above it. It can also be caused by a discontinuity forming in the upper atmosphere over a very wide area.

Good descriptions and sketches of the various forms of mirage and the effects of abnormal refraction are always of interest, especially of the more striking forms, such as a well- developed superior mirage. Unusual phenomena should be carefully reported, such as the apparent discontinuity of distortion of the horizon line that has been occasionally seen, also lateral mirages and the complicated mixed mirages of the Strait of Messina, known by the name, of Italian origin, of 'fata morgana'. When lights are seen at abnormal distances, the normal distance of visibility should

be given. In all observations of abnormal refraction and mirage, the temperature of air and sea, the type and amount of cloud present, and the direction and force of the wind should be noted.

Photography may be helpful in resolving controversial questions such as that of the possibility of lateral mirage in the free atmosphere and that of mirage magnifying (or diminishing) laterally and vertically simultaneously. It is particularly useful if the characteristics of the camera and its lens system, and also in the latter case those (distance and dimensions) of the objects whose mirage images are observed, are known and logged.

In a foggy atmosphere an observer, standing with his back to the sun, when this is at low altitude, will sometimes see the shadow of himself, or of his head, thrown upon the fog, together with coloured rings of light surrounding the shadow. The phenomenon was first noted on the Brocken mountain in Germany but it is not confined to mountain districts and it is most common in Arctic regions, where it is seen on every occasion of simultaneous sunshine and fog.

The coloured rings are now usually known as a 'glory'. A typical series of colours seen in a well-developed one is as follows. There is a general whitish- yellow colour round the shadow, surrounded by rings of colour in order outwards: dull red, bluish-green, reddish-violet, blue, green, red, green, red. A white rainbow at a considerable distance outside the glory is sometimes also seen.

The shadow of the observer on thick fog may be seen at night if there is a bright artificial light behind him.

The various red or orange colours ordinarily exhibited by the sun, moon and some other astronomical bodies when near the horizon are generally caused by these bodies being viewed through a great thickness of the dust-laden lower atmosphere, which absorbs most of the sunlight of shorter wavelengths, leaving the longer ones, mainly yellow and red, to come through.

Occasionally at twilight the moon appears to be of a greenish colour, usually a pale greenish-blue or a pale apple-green colour. This is an effect of colour contrast when the twilight hues of the surrounding sky are brighter than usual, either purplish or reddish, or when the moon is near or covered by thin, brightly tinted cloud.

Coloured suns or moons, not an effect of colour contrast, are sometimes seen. This phenomenon may be produced by dust or smoke haze in the lower atmosphere, e.g. a scirocco laden with dust from the Sahara may give a blue sun or moon in the Mediterranean, and a similar colour may be given in the region of extensive bush fires. The phenomenon may also be produced by volcanic dust at high atmospheric levels. Blue and green moons were observed on many occasions after the great eruption of Krakatoa in 1883 and the sun assumed many different and often quite brilliant colours. Shades of red and copper, green, golden-green, blue, both silvery and leaden, were seen on various days in different localities.

Coloured suns and moons were seen over much of western Europe between 26 and 30 September 1950. These were produced by the smoke from an extensive forest fire in Alberta, Canada which began on 23 September. The sun's colour was observed in different places as steel-grey, deep blue and purple.

A corona consists of one or more coloured rings round the sun or moon as centre, when either of these bodies is covered with middle or lower cloud thin enough to allow the greater part of the light to come through. It is distinguished from a halo by its smaller size and different colouring, as explained below. A fully developed corona shows a bluish-white or yellowish glow, usually 2º or 3º in diameter, round the sun (or moon). Outside this is a brownish-red ring. The inner glow and the brownish ring together constitute what is called the aureole. Outside this are coloured rings, in the opposite colour sequence to that of a halo, namely violet or blue nearest the sun and red farthest out. Sometimes the whole of this colour sequence is repeated outwards a second or, on rare occasions, even a third or fourth time. A corona showing the outer coloured rings is comparatively infrequent, but the aureole alone is the commonest of optical meteorological phenomena and is formed, at any rate partially, whenever broken cloud edges of cumulus, stratus or stratocumulus pass over the sun or moon.

While the radii of the various babes are constant, that of a corona varies on different occasions, being dependent on the size of the water drops in the cloud. The outside radius of a fully developed corona is usually much smaller than that of the 22º halo, and is generally between 5º and 8º. After great volcanic eruptions, when fine dust is suspended at great heights in the atmosphere, an aureole comparable in size with the 22º halo has been seen; it is known as Bishop's ring.

If the drop size varies from place to place in the cloud the corona will have an irregular circumference, this being the transition to 'iridescent cloud' (see below), with completely irregular patches of rainbow colour.

Faint coronae are visible round the bright planets, Venus and Jupiter, and also Mars when this is sufficiently bright, providing the cloud is very thin. They may sometimes be seen round the brightest stars, especially if binoculars are used.

A yellowish blur 2º or 3º in diameter is often seen round the sun or moon and is sometimes formed by higher cloud than that which normally gives coronae. Although it has a fairly sharply defined circular edge it must not be regarded as an aureole unless bounded by the characteristic brownish-red ring.

In certain circumstances the sun or moon may show a halo and a corona simultaneously.

The name 'corona' is also given to the outer part of the sun's atmosphere (see page 144); this is directly visible only during a solar eclipse and is distinguished by the term 'solar corona'.

The electrical phenomenon known as Corposant or St Elmo's Fire is not infrequently observed at sea during squalls and thunderstorms. It is a luminous apparition seen at the extremities of masts and sometimes on the stays, aerial, jackstaff or other parts of the ship. It may appear as a brush discharge of radiating streamers several inches long, or as luminous globes, a number of which are sometimes seen along the aerial. At other times a structureless glow envelops an elongated object, such as a mast or an aerial. St Elmo's Fire is usually bluish or greenish in colour, but a violet glow has been reported and sometimes the colour is pure white.

The word 'crepuscular' means 'associated with twilight'. Occasionally, soon after sunset, the clear sky appears to be divided into lighter and darker rays by lines diverging from the position of the sun below the horizon. The lighter rays are those illuminated by sunshine; they are usually coloured pink, but may, on different occasions, show some shade of red or orange. The darker rays are shadows, from which the sunlight is cut off by clouds near or just below the horizon or by the irregularities of hills and mountains on the horizon. They appear greenish by contrast with pink rays. (See photographs on page 93.)

As the light rays come from the sun, and so are practically parallel, their apparent divergence is an effect of perspective. In favourable circumstances the light rays and shadows extend right across the sky and appear to converge, by perspective, to a point a little above the eastern horizon. These 'anti-crepuscular rays' are generally ill defined.

It is not necessary to record this phenomenon in the logbook unless it shows some feature of special interest, such as unusually distinct colouring or a well-defined convergence to the eastern horizon.

On rare occasions one or more bands have been seen extending up into the sky, from the western horizon, at a later stage of twilight. They appear of a deep blue colour, darker than the general blue of the sky, and are probably shadows of mountains well below the horizon. It is of interest to record these observations.

There are two other allied phenomena which are frequently seen and are of no special interest, unless some unusual feature is observed. The first consists of pale blue or whitish rays diverging from the sun in the daytime when it is behind cumulus or cumulonimbus cloud. The rays are sharply defined and separated by deep blue bands, which are the shadows of parts of the irregular cloud edge. The second is associated with stratus or other cloud obscuring the sun. If there are small gaps in the cloud, sunbeams pierce these, directed more or less downwards, and are rendered luminous by mist or dust in the air. This is popularly known as 'the sun drawing water'.

Dust from the land may be blown over the adjacent sea by high winds, but not normally in appreciable quantity. In special regions, e.g. the Red Sea, sand-or duststorms are not infrequent and are sometimes severe.

Desert dust or sand may be carried up to high levels of the atmosphere and finally be dispersed over so great an area as not to form any perceptible deposit on falling. The desert dust from Australia carried north-westward by the south-east monsoon reduces visibility over the East Indies region but is not observed as dustfall.

On the other hand, falls of fine reddish or brownish dust from the Sahara, carried by the trade wind, are experienced over a large area of the eastern North Atlantic adjacent to the coast of Africa, centred roughly on Cape Verde Islands. At times this deposit may lie quite thickly on board ship. Visibility in this area is often poor; not infrequently the sun appears blood-red and at night all but the brightest stars at high altitudes are obscured.

Examples of dustfall. Considerable or heavy dustfall may be experienced after a great volcanic eruption. Dust from the eruption of Krakatoa in l883 was collected on board ship in the Indian Ocean at a distance of 1000 nautical miles. After the eruption of Hekla in March 1947, dust was similarly collected at a distance of 450 n.mile. In August 1966 the dust from the eruption of Mount Awu in the Sangihe Islands covered the decks of a ship 225 miles away in the Celebes Sea.

In more recent times, volcanic dust from the eruption of Mount Pinatubo in the Philippines in June 1991 was collected on board m.v. British Skill when more than 450 n.mile south-west of the volcano's position; and after the eruption of Mount Hudson in Chile on 17 August 1991, m.v. Remuera Bay experienced a grey-green haze and heavy dustfall when passing west of the Falkland Islands nine days after the eruption and more than 500 n.mile distant from the volcano.

At sunset, the small segment of the upper part of the sun's disc, which is the last to disappear, may turn emerald-green or bluish-green at the instant of its setting. The phenomenon thus usually lasts only a fraction of a second, which is the reason for calling it the 'green flash', but longer durations of the colour are occasionally seen.

Sun. The green flash is not always seen and when it is seen it is not always equally brilliant. It can range from a green of extreme brilliance and purity, conspicuous without optical aid, down to a trace of grey-green coloration observable only with binoculars.

The green flash is produced by the last rays of sunlight emanating from the upper limb of the sun, at sunset, being refracted before reaching the observer's eye. The shorter waves which appear as violet, blue and green light suffer greater refraction than the orange and red longer waves of the white sunlight. The fringes of the upper limb cannot usually be seen while the main body of the sun is still above the horizon, as the general sunlight is too strong, but when most of this is cut off by the horizon they spring suddenly into view. Normally, only the green fringe is seen, the light of still shorter wavelengths usually being scattered by its horizontal passage through the lower atmosphere. The flash is, however, occasionally seen as a blue one, or as green quickly changing to blue. On very rare occasions the violet colour has been seen.

The green colour occasionally appears in other ways. Sometimes when refraction is marked, and the sun's disc is perhaps distorted, the use of shaded binoculars will show that the upper limb appears to be 'boiling', giving off shreds or tongues of green 'vapour'. Occasionally the sun's upper limb has been seen with a narrow green rim when half or more of the disc remained above the horizon.

A sea horizon is not essential for observing the green flash; it may be equally well seen when the sun sets behind a distant land surface. It has also often been seen when the upper limb sinks below a bank of hard-edged cloud at low altitude, and if there are several parallel bars of cloud in clear sky the phenomenon may be seen more than once on the same evening. When the lower line of the sun appears from behind cloud near the horizon the converse phenomenon, the 'red flash's has sometimes been seen.

Moon and planets. The green flash occurs also with the moon, but has seldom been observed, presumably because it is fainter and rarely looked for. On the other hand, it has been frequently seen at the setting of the bright planets Venus and Jupiter, and an observation of a blue flash from Venus is on record. Many interesting varieties of phenomena may occur before these planets set, the observation usually requiring binoculars. Colour changes may be seen, usually between white, red and green, or two images may appear of the same or different colours. The planet may exhibit slow 'shimmering' movements, obviously due to abnormal refraction.

Observing conditions. The most favourable conditions for seeing the green flash, at any rate brilliantly, is probably some degree of abnormal refraction, whereby the vertical extent of the colour separation described above is greater than that produced by normal refraction. In addition, the green flash is most likely to be seen when the air is relatively dust-free, and without mist or haze, so that the sun remains brighter and less red than usual at low altitudes. The green flash has been well observed at sunrise, but less frequently, perhaps because it is less often looked for. Also, owing to its short duration the phenomenon is liable to be missed unless the exact spot at which the sun will appear is known.

The green flash has sometimes been called, rather inappropriately, the 'green ray'. It will be obvious from the remarks made above that it exhibits a considerable variety of appearances at different times. Further observations, giving as much detail as possible, will be very useful in increasing our knowledge of the interesting phenomenon and the conditions most favourable for its appearance.

Other phenomena involving green coloration of the sky in the vicinity of the sun at the moment of sunset are occasionally seen, and observations of these are also of interest, as they exhibit much variety. Some examples are (a) a momentary ray of green light shooting up into the sky, sometimes to a considerable altitude at, or just before, the final instant of sunset, (b) an appearance resembling a rapidly rotating green searchlight beam, (c) a transitory appearance as of green mist in the sky above the setting sun.

These phenomena may show colours when formed by refraction of the light from the sun, but the rarer halo phenomena produced by the light of the moon are usually white. For convenience the source is usually referred to as the Sun. The many different kinds of halo which have been observed may be described by reference to Figure 27. This is a composite diagram made up from a number of drawings of an unusually complete halo display seen at about midday on 6 March 1941 in various localities in the west Midlands.

The four rings described below (three babes and the parhelic circle) may, under very favourable conditions, appear complete but they are more frequently incomplete. Parts of these rings, together with the arcs and mock suns , may at times be seen with no apparent connection with one another; the sky may then assume a very strange appearance.

An unusually complete halo display observed at about midday on 6 March 1941 in the west Midlands; composite diagram based on observations from various localities. Appearances due to refraction, which may he brilliantly coloured, as on this occasion, are shown black; appearances due to reflection, which are always white, are shown by the finer lines. The outer circle, H, is the complete horizon, with Z,the zenith, in the centre; S is the Sun, P the anthelion, A, B, C, D and E are parhelia or mock suns; F is the 22º halo, G the 46º halo; J and K are upper and lower arcs of contact of F; L is the parhelic circle or mock-sun ring (parallel to the horizon), M the arc through the mock suns at 120º (usually a pair of arcs not joined in the middle), and N the oblique arc through P (one of a symmetrical pair which may sometimes be seen together).

The halo phenomena predicted by standard theory are symmetrical about the solar vertical, apart from possible suppression in certain parts of the sky by the absence of suitable crystals in that direction. Certain very rare exceptions are apparently due to orientation of crystal axes in a direction somewhat tilted from the vertical.

This is the most frequent halo phenomenon and appears as a luminous ring, F, in the figure, with the sun or moon, S. as centre, and having a radius of 22º. The space within the ring appears less bright than that just outside. The ring, if faint, is white; when more strongly developed it shows coloration; the edge nearest the sun is red and this is followed by yellow and, in some rare cases, a green or violet fringe can be detected on the outside.

The angle of 22º is the angle of minimum deviation for light passing through a prism of ice with faces inclined at 60º and this halo is probably due to the refraction of light through hexagonal prisms among ice crystals in cloud.

Among the phenomena which, from their manner of information, can be seen only as arcs, are the so-called arcs of contact. Two of these are shown in Figure 27 – an arc of upper contact, J, and an arc of lower contact, K. The arc of lower contact is rarer than the upper, and contact arcs which occasionally appear at the sides of the halo are very rare.

When the sun is low the arcs of upper contact appear with their convex sides turned towards the sun. The points of contact with the halo are brightest, and sometimes all that is to be seen of these arcs are local brightenings of the halo at these points.

When the sun is high the arcs of upper and lower contact may appear concave to the sun. Occasionally at these higher solar elevations, the ends of the arcs are joined to form a circumscribed halo which is approximately elliptical when the sun is high enough. Other arcs, somewhat similar to the arcs of contact, but for most solar elevations standing just a little further out from the sun, and convex or concave to it, are occasionally seen. These are called Parry arcs, and there are generally two types above, two below, the sun.

Parhelic circle (the mock-sun ring) and the parhelia ('mock suns' or 'sun dogs’) A number of halo phenomena are to be observed on a circle centred on the zenith and passing through the sun. It is evident that, the higher the sun, the smaller is this circle, and that when the sun is very low, the circle appears as very nearly straight (that is, as part of a great circle), and parallel to the horizon. Occasionally the whole circle appears as a white ring, the horizontal or parhelic circle or mock-sun ring, or substantial portions of such may be seen. The commonest phenomena on the circle, however, are the parhelia, mock suns, or sun dogs, sometimes distinguished as the parhelia of 22º. With low sun, these bright spots stand on the halo of 22º, but they gradually move out slightly as the solar elevation increases, and for very high sun cannot be seen at all. They can be very luminous, especially at low sun, and of brilliant colours, especially at rather higher sun when these parhelia are smaller. Even with low sun, the red, on the side nearest the sun, is prominent. Sometimes most notably when the sun is moderately low the parhelia have whitish tails, extending some way, outwards only (that is away from the sun), along the parhelic circle.

Other lightspots occasionally seen on the parhelic circle are the white anthelion or counter- sun at 180º azimuth from the sun, and perhaps slightly commoner are at least one kind of 'paranthelia' lying between the parhelia of 22º and the anthelion. Of the paranthelia, the best attested are the white parhelia of 120º, standing at this azimuth from the sun, though there seem to be other, rarer classes (coloured according to theory).

The corresponding phenomena produced by the moon are called 'paraselenic circle', 'paraselenae', 'parantiselenae' and the 'antiselene': the last three can be called 'mock moons'.

Various arcs are to be seen crossing the parhelic circle, often obliquely, at all these lightspots. Those at the parhelia of 22º are the lateral tangents arcs, already described, of the halo of that radius. For certain modes of tipping of the crystals there may be several of these. The best attested are the Arcs of Lowitz sloping down from the parhelia of 22º to contact with the 22º halo. Occasional observations of parhelia tilted in this sense at higher solar elevations may perhaps be incipient forms of some of these arcs.

There may be more than one kind of paranthelic arc (through a paranthelion). Since there is overall symmetry about the solar, and hence also about the anthelic, vertical, the very rare anthelic arcs generally occur as pairs of mirror-image oblique arcs crossing at the anthelics. Many such arc-pairs are theoretically possible, and at least two pairs of anthelic arcs have been seen simultaneously. Occasionally looped extensions, passing a good way across the sky, have been seen to link the two arms of a pair. In theory, some anthelic arcs may be coloured in regions remote from the anthelion. Similar white arcs, the heliac arcs, have on very rare occasions been seen intersecting at the sun.

These are fairly common, at any rate at sunrise and sunset. They generally taper upwards, sometimes to at least 20º above the sun. At sunset they may be entirely red, but usually they are white, occasionally blindingly so, and on rare occasions may show a marked glittering effect,

This is a halo phenomenon produced by reflection of sunlight on ice crystals in clouds. It appears vertically below the sun in the form of a brilliant white spot, similar to the image of the sun on a calm water surface. It is necessary to look downward to see the undersun; the phenomenon is therefore only observed from aircraft or from mountains.

Another whole class of halo phenomena similar to that comprising the 22º halo, parhelia, arcs of contact and Parry arcs, but rarer, is to be observed at an angular distance of 46º, or a little more, from the sun. These phenomena require crystals with faces at right angles. The true halo of 46º is rare, and indeed its real existence has been doubted. Many observations claimed as being of such may actually be of its supralateral tangent arc (see below).

The least controversial of this group of phenomena are the upper and lower circumzenithal arcs (also sometimes called respectively simply ‘the circumzenithal arc’ and ‘the circurmhorizontal arc’ respectively): they appear to lie in horizontal planes, like the parhelic circle.

The upper circumzenithal arc (brightly coloured, with red on the outside and violet on the inside) is a rather sharply curved arc of a small horizontal circle near the zenith; the lower circumzenithal arc is a flat arc of a large horizontal circle near the horizon. The upper arc occurs only when the angular altitude of the luminary is less than 32º; the lower arc occurs only when the angular altitude of the luminary is more than 58º. The upper arc touches the large halo, if visible, when the angular altitude of the luminary is about 22º; the lower arc touches the large halo when the angular altitude of the luminary is about 68º. The arcs become increasingly separated from the large halo as the angular altitude of the luminary departs from the above values. Circumzenithal arcs may be observed without the large halo being visible.

Witnessed from m.v. Nova Scotia, Captain N.R. Land, St John (N.B.) to Liverpool. Observer, Mr A.C. Herdan, 3rd Officer. 'l October 1965. Position 43'16 N, 66015'W. The halo complex was clearly seen from 1200-1600 GMT. The radius of the inner halo was 21º26 and that of the partial outer concentric halo was 46º30'. Two wing-shaped arcs, each subtending an angle of about 54º, crossed the outer concentric halo, meeting at the centre of their span directly above the sun and in contact with the upper edge of the inner concentric halo. At the point of contact (A) a brilliant spectrum could be seen subtending an angle of at least 1 ¹/₄º About 1530, the most vivid period, two more haloes were seen. One was a white arc which would have stretched right across the halo complex, passing through the position of the sun, if it had not been rendered invisible by the glare. The other was a small, but vivid, inverted half halo of about 6º radius; it was in contact with the inner halo, its upper edge crossing the latter's lower limb (B). At this point also vivid coloration was seen. Altitude of sun: 21º50; bearing 094º. Cloud, small amounts of Cirrus and Altostratus.'

Two arcs of contact of this halo are known, the supralateral and infralateral tangential arcs of 46º. They are in contact, on the solar vertical, with the (upper) circumzenithal and the circumhorizontal arcs respectively. Also, they are bitangent or bitangential arcs in that, in the most general case, they contact the 46º halo (or at least, its theoretical position) at two points each and they are lateral tangential arcs in that these points of contact are not generally on the solar vertical. For the supralateral arc, when the solar elevation is less than 22º, the points of contact are on the upper sides of the halo, and they move towards its top as that elevation increases. At that critical solar elevation, the two contacts merge at the top of the halo, and for higher solar elevations up to 32º (when it ceases to be formed) the arc stands clear of the halo. At least in their upper parts, this arc and its parent halo are so similar in shape and so close together as to be liable to the confusion noted above.

For solar elevations less than 58º, the infralateral arc consists of two separate arcs concave to the sun and touching the 46º halo on its lower sides, nearer the bottom of the halo the higher the sun. At this critical solar elevation the lower ends of these two arcs merge on the solar vertical. At a solar elevation of 68º the two points of contact merge on the solar vertical, and for higher sun the single arc is detached from the position of the halo of 46º.
The observation and recording of haloes
High latitudes, especially the polar regions, are the most favourable for frequent and brilliant displays of halo phenomena, which can be formed not only by cirrostratus cloud, but also by ice fog. Many fine displays occur, however, in temperate latitudes, where the late spring is an especially good season.

Cirrostratus is the most favourable cloud for the production of halo phenomena; the thinner and more uniform its texture the better. On the most suitable occasions, the blue sky is only dimmed with a uniform milky appearance. When the cloud is thicker in some places than others, and especially when wisps and streaks of cirrus are mixed with it, not only are the phenomena less distinct but straight or curved lines of cloud may be mistaken for additional halo phenomena.

When thin cirrostratus is present and one or more of the commoner halo phenomena are well seen, the prospects of seeing some of the rare halo phenomena are good and a careful general look over the whole sky may result in something else being seen. Attention should chiefly be concentrated on the following regions (a) that surrounding the sun up to a radius of at least 46º, (b) a belt of the sky, at the same altitude as the sun, all round the horizon, (c) the overhead sky, with the zenith as centre.

A halo phenomenon is thus identified by its position in the sky; its appearance is of secondary importance, though, in some cases, this helps in the identification. The most essential part of a halo observation is therefore the determination of its position by angular measurement with reference to the sun (or moon) or, in appropriate cases, the horizon or the zenith. Most of the rarer phenomena can only thus be identified with certainty.

The altitude of the sun, to the nearest degree, should also always be given, since this affects the precise position of certain halo phenomena, and in some cases determines what phenomena it is possible to see at the time. The radius of the relatively well-defined inner edge of any halo, or part of a halo, centred on the sun should be measured in degrees from the sun's centre. Record what is measured as the radius of any halo, whether to an inner or outer edge or a brightest point. The sharpness of the edges may also be worth noting. In the case of arcs situated vertically above the sun such as the circumzenithal arc, the distance of the lowest part of the arc from the sun is all that is required. It is useful, however, to estimate the extent of any such arc as a fraction of the small circle of which it forms part.
The mock-sun ring
This is identified by its parallelism to the horizon, at the sun's altitude. It is probably worthwhile to check the elevation of this circle at one or two points other than the sun, as it has on rare occasions been seen tilted, and such checks on suspected fragments of the circle will render their identification more certain. A phenomenon situated on it, such as the anthelion or other bright spot, or the point of intersection of an arc with it, is measured in the form of azimuth distance from the sun. Even for the parhelia of 22º, checks on the theory are desirable, and this measurement will confirm the identity of any suspected anthelion. Most importantly, it will give insight into the precise mode of formation of any paranthelia seen.

The above statement should be sufficient to indicate to the observer the lines on which he should proceed. The most difficult cases are certain abnormal phenomena such as are shown in Figures 28 and 29. The diameter of any halo not centred on the sun or moon could be measured by sextant; the altitude and azimuth of the

Witnessed from m.v. Koraki. Captain H.C. Townend. Tauranga to Melbourne. Observer, Mr M.J.C. Orr, 2nd Officer. '28 May 1967. Position 34º34'S 171º42'E. The 22º halo was first seen at 1415 GMT when the moon was at an approximate altitude of 60º and bore 090º. At 1450 the tangential arcs appeared, weak at first but gradually gaining in intensity in the next 10 minutes. The complex remained visible for about 20 minutes, after which time only the 22º halo was seen. A thin layer of Cirrostratus to Altostratus was present and also some Cumulus.'
estimated centre of the halo would then give its position. The position of any detached arc could be measured by taking the altitude and azimuth of each of the two ends, and of the point on the halo equidistant from these.

Having established the position, any point of special interest should be noted, such as an exceptional degree of brightness or colour, variations in brightness in different parts of a halo, or a halo appearing elliptical instead of circular, etc. In the case of the rarer phenomena, the fullest possible information should be recorded, preferably accompanied by a sketch, on which all angular measurements are shown. In sketching halo phenomena the size of the sun (or moon) is usually exaggerated, sometimes very greatly. Even in landscape paintings by well- known artists, the same thing usually occurs. The discs of the sun and moon are about half a degree in diameter and therefore only about one- ninetieth of the diameter of the common halo of 22º radius.
Other halo phenomena
There are a few other phenomena not included in the previous pages, e.g. various forms of cross, centred on the sun or moon, are occasionally seen (Figure 30). The vertical arm is usually formed by part of a sun pillar and the horizontal arm by a short portion of the mock-sun circle. Haloes of other radii are occasionally seen (often several of them simultaneously), and so are rarer arcs, but the 'halo of 90º' sometimes referred to is probably a false association of reports of a number of distinct phenomena. One such observation is shown in Figure 28 in which is seen the ordinary 22º halo, with an arc above, possibly part of the 46º halo, but more likely its supralateral tangent arc.

The ideal method of recording halo phenomena is by colour photography, since, if the characteristics of the camera and its lens are known, the material is available for later analysis. An all-sky camera, if available, is particularly valuable.

Ideally, observed halo phenomena should be represented on the surface of a sphere or hemisphere or part thereof, but this is hardly ever practical: a flat sheet of

paper is all that is available. In the representation on such of halo phenomena confined to a small area of sky, it is natural to represent any great-circle arc (or near-great small-circle arc) as a straight line, and any parallel arc as a parallel line. Unfortunately, the only real meaning of 'parallelism' in terms of the celestial-sphere geometry of halo phenomena is 'concentricity' (that is, only a small circle can be 'parallel' to any great circle) and this can lead to problems of representation.

For instance, if, with fairly low sun, the horizon be represented by a straight line, it is natural to represent the parhelia at the same distance from it as the sun is shown. But at higher sun, this obviously fails to represent adequately the fact that the parhelic circle, on whose position these phenomena are located, is a circle, centred on the zenith. If in any doubt, the best convention is to select a point to represent the zenith, describe as much of the horizon as is required as a circular arc centred on that point, and similarly centre circles or circular arcs indicating the positions of the parhelic circle and circumzenithal or circumhorizontal arcs on that point. It is unlikely that similarly drawing every halo observed to be circular (even great- circular) as a circle centred on the point selected to represent its centre will give rise to difficulty if the radii are carefully drawn, albeit that only a very artificial projection will have these characteristics. This has been done in Figs 27 and 28 to represent the arcs associated with the halo of 46º.

Patches of delicate, but often vivid, colouring are occasionally seen at any time of the day on altocumulus and other middle and high clouds, often covering quite a large extent of cloud. It may form a very beautiful spectacle, especially if the sun is hidden from the observer's view by lower cloud. Red and green are the most common colours, but others, such as lilac, may be seen. Sometimes the colours lie in bands parallel to the edge of the cloud, but often they form an irregular mosaic, delicately shading into one another. The colouring resembles that of coronae, but the bands of colour do not form concentric circles with the sun as centre. Sometimes a number of coloured patches may be seen along a straight line passing through the sun.

Iridescence is usually seen on cloud near the sun or within about 30º of the sun, but may occur at greater distances. It seems to be most frequently observed on cloud that is in the process of either formation or evaporation. The colouring is not normally seen after sunset or before sunrise, but brilliant iridescence, continuing after sunset, or appearing before sunrise, may be seen on a very rare high form of cloud, called 'mother-of-pearl cloud'.

If the observer is in doubt as to whether he is seeing ordinary sunset cloud colouring, or iridescent colouring towards the time of sunset, it should be remembered that the former may cover large areas of cloud, or many isolated clouds, with no colour. Iridescence, on the other hand usually shows much smaller areas of different colour on one cloud and the coloration is purer and more prismatic, in this respect resembling the colours of the rainbow.

When seen, remarks on the nature and extent of the colouring, the type of cloud and the approximate angular distance from the sun will be useful.

Lightning varies in colour on different occasions: it is normally white, with perhaps a bluish tinge. Sometimes it is quite a bright violet. Other colours seen are reddish-white, yellowish- white, mauve and blue.

(a) Inequalities of brightness in different parts of the path, known as chain or beaded lightning, from the impression left on the eye.

(b) Rocket lightning, so called from the relative slowness of the flash, allowing the progressive lengthening of the streak to be seen.

A high frequency of visible flashes sometimes results from more than one storm in different directions being operative at the same time, so that at night there is almost continuous illumination of the sea or landscape. Such a lightning rate has been known to persist for several hours, but this is very unusual.

Occasional reports of ships being struck by lightning are received, but this event is probably of much less frequent occurrence than in the days of wooden sailing ships. Descriptions of the effect on the ship and on the compasses (see page 151) will be of interest. Observations of recent years show that in nearly all cases the foremast or fore part of the vessel is struck.
Ball lightning

The special form known as ball lightning resembles a ball of fire, either falling from a cloud or moving more or less horizontally. It usually lasts only a few seconds and may disappear noiselessly or with an abrupt clap of thunder. Ball lightning has been seen at close range and it has sometimes passed into or through a building. Rare forms of lightning have also been seen shooting upwards from the top of cumulonimbus cloud, in various branching or rocket- like forms.

Observations of ball lightning. Careful observations of this uncommon, but not extremely rare, form are specially desired. Observers should log the following points when ball lightning is seen:

Observations of ball lightning forming are particularly rare. A labelled drawing is always helpful. There are almost no acceptable photographs of ball lightning. Reports of damage done by ball lightning are useful, but care must be taken to distinguish this from damage done by an ordinary lightning strike.

The normal appearance of a bright rainbow is as follows. The chief or primary bow shows the sequence of colours, violet, indigo, blue, green, yellow, orange and red, the red being on the outside or top of the bow. In contact with the inside of this bow, one or two fainter 'supernumerary bows' can frequently be seen with the colours in the same order, the first inner bow being much fainter than the primary bow and the second fainter still. Supernumerary bows do not, however, show the full range of spectrum colours; they are essentially red, or red and green, though other colours may be seen. In cases of exceptionally brilliant rainbows up to five supernumerary bows may be seen.

Secondary rainbow. Concentric with the primary bow, but 9º outside it, is the secondary rainbow, in which the full range of colours appear in the reverse order, red inside and violet at the top or outside. The primary bow is formed by means of one internal reflection in each raindrop; the secondary bow is fainter, being produced by two such reflections. The sky between the primary and secondary bow is rather darker than that inside the primary bow, or the general sky in the neighbourhood. The secondary bow is commonly seen, but if the primary bow is faint the secondary one may not be visible.

Both the primary and secondary bows are seen when the observer has his back towards the sun. The sun, the observer's eye and the centres of the circles of which the primary and secondary rainbows form arcs, are always in a straight line, so that the azimuth of the highest part of the bow is 180º from the sun's azimuth. The normal radius of the arc of red light of the primary rainbow is 42º, of the violet arc 401/4º; in the secondary bow the radii are 51º for red light and 54º for violet light, all the values given being approximate. Hence the normal breadth of the primary bow is about 13/4º and that of the secondary bow about 3º It also follows that with the sun at an altitude of 42º the uppermost point of the primary bow is on the horizon, its centre being 42º below the horizon, and hence no primary bow can be formed if the sun's altitude exceeds 42º. Similarly no secondary rainbow can be formed if the sun's altitude exceeds 54º. Consequently rainbows are mainly morning and evening phenomena; nearer midday, if seen at all, the arc of the bow is shorter and the altitude small. Thunderstorm rain passing away from the observer gives the most favourable circumstances for the production of bright rainbows.

When the observer is at ground level and the rain cloud is distant, the rainbow arcs are always less than semicircles, unless the sun is on the horizon, when they form semicircles. When, however, the rain is near, and especially if the observer is in an elevated position, such as on the bridge of a ship, the bows will be greater than a semicircle and may even form complete circles. Several accounts have been received of bows forming complete circles as far as the waterline on each side of the ship.

One of the halo phenomena, the circumzenithal arc, may show bright rainbow colouring, but is always in such a position that the observer must face the sun to see it.

Rainbow colouring. Rainbows do not always show the same colouring. The colours seen, and their relative width and intensity, vary according to the size of the raindrops producing the bow. The colours are most brilliant and best defined with very large raindrops such as occur in thunderstorm rain. With fairly large drops, vivid violet and green may be seen, and also pure red, but little or no blue. With smaller drops the red weakens and with still smaller ones the green goes, leaving only the violet. Just before sunset, when the sun is red in colour, especially in autumn and winter, an all-red rainbow may be produced.

White rainbow. If the raindrops are extremely small, as in the case in some cloud and in fog, a white rainbow may be formed. Such a bow is called a 'fog-bow' or 'Ulloa's Ring'. In all rainbows there is some overlapping of the colours; in a white rainbow the overlapping is so complete that white light is reconstituted. For a white rainbow to be seen, the observer must be near the cloud or near or in the fog.

Lunar rainbows are formed in the same way as solar ones, but are considerably rarer, having regard to the comparatively short periods that a bright moon is above the horizon. A lunar rainbow is usually fainter than a solar one and it is not always possible to distinguish colour; the appearance is then whitish. Quite frequently, however, colour may be observed; more rarely the whole sequence of colour can be seen. Secondary and supernumerary lunar rainbows are very rarely seen, on account of their faintness.

These are seen occasionally on calm days when a sheet of water lies in front of or behind the observer standing with his back to the sun. Such bows are formed by rays of light illuminating the falling raindrops after reflection at the surface of the sheet of water. The centre of a reflection rainbow is thus as high above the horizon as the sun, or the same angular distance above, as the centre of the direct bow is below the horizon; consequently the arc, when complete, exceeds a semicircle. The direct and reflection rainbows intersect on the horizon, and the colours have the same sequence.

The observer who wishes to make useful observations of normal rainbows should record the colours seen, in sequence, with an indication of their relative widths and intensities. If supernumerary bows are seen below the primary bow, the number of these and their colouring should be noted. If the secondary bow is unusually bright it is worthwhile looking for supernumerary bows just above it; these have rarely been seen on account of their faintness. An additional primary bow may be seen when the sea is sufficiently calm to give a reflected image of the sun in the sea, which acts as the light source for the bow. The position of this bow with regard to that formed by the sun itself varies with the sun's altitude. The secondary bow from the sun's reflected image is almost always too faint to be observable.

Abnormal bows, or arcs of bows, perhaps intersecting the normal bows, and sometimes white in colour, have occasionally been seen and it is of special interest to record these as fully as possible, since no explanation has yet been found for some of them. They sometimes meet the horizon at the same point as one of the normal bows. In such cases the sequence of colour, or the absence of colour should be noted. It is essential to give angular measurements of such bows, in the form of azimuths of the ends of the bow or arc, in which case the sun's azimuth should also be given. If a normal bow is also seen, the difference in azimuth between the points where the normal and abnormal bows meet the horizon will serve to establish the position of the latter. If an abnormal bow is seen concentric with the normal primary or secondary bow the difference of altitude of the bows at their highest point should be given.

Scintillation, or twinkling, is the more or less rapid change of apparent brightness of a star, accompanied also at relatively low altitudes by colour changes. It is due to minor changes in the refractive power of the atmosphere. The amount of twinkling is always greatest towards the horizon and least in the zenith. The general amount varies considerably on different nights, so that at the zenith twinkling may be considerable, slight or entirely absent. Nights without appreciable twinkling towards the horizon are rare. When the changes of brightness are small the fluctuations are slower; in proportion as they are greater they become more rapid.

Colour change is usually shown by stars at altitudes not exceeding 34º; it never occurs at altitudes greater than 51º. The brightest stars, e.g. Sirius, at low altitudes show it most and, in favourable conditions, the changes may be very striking, the star flashing blood-red, emerald- green, bright blue, etc.

Scintillation is also observed in the case of terrestrial lights. The shimmering seen near the ground on a hot day is akin to it.

The bright planets do not usually appear to twinkle, as they have discs of definite size, although these are not visible without optical aid. Each point on the disc twinkles independently of the others, so that on the average the light is steady. The planet Mercury, only seen in twilight and at relatively low altitudes may, however, be seen to twinkle because of the small size of its disc, and, exceptionally, other planets at very low altitudes may exhibit some twinkling.

The relative degree of twinkling in different parts of the world, e.g. in temperate as compared with tropical latitudes, is not very well known and any information bearing on this will be of interest. It is probably greatest in temperate latitudes, which are subject to the passage of depressions.

The light of the sky in daytime is due to the illumination of the atmosphere by sunlight. The molecules of air exert a selective action on the colour constituents of sunlight, scattering mainly blue rays in all directions and letting the others pass on.

Dust is always present in greater or less degree in the atmosphere, and in certain states of weather larger particles are present in the lower part of the atmosphere. The presence of dust tends to weaken the blue of the sky, because each particle reflects the whole of the white light. The greater the number of particles and the larger their size the more the sky becomes whitish-blue. For the same reason the cloudless sky is always whiter near the horizon than at higher altitudes. After heavy rain, such as that due to the passage of a depression, the larger dust particles have been washed out of the air and the sky is often a very deep blue.

The sky is often whitened within the region of smoke pollution from a large town. Natural dust from desert sources, at higher levels in the air, also has the same effect, e.g. the white skies seen in the region of the East Indies in the south-east monsoon, caused by dust from the Australian desert. The dust from great volcanic eruptions may whiten the sky for weeks or months afterwards, over more or less considerable areas of the globe. After the Krakatoa eruption the colour of the sky at various times of the day in equatorial regions was described as white, smoky, yellowish or reddish.

The unclouded sky may also be whitened by what is known as 'optical haze', which also makes distant terrestrial objects indistinct. This occurs on hot days and is the result of innumerable little convective uprisings of air, causing confused and variable refraction of light. The shimmering of terrestrial objects on a hot day also results from the same cause.

The sky may sometimes be covered by a layer of cirrostratus, so thin and uniform as not to be visible as cloud, but sufficient to dim the blueness, giving the sky a milky appearance.

A somewhat dirty green coloration of clear patches in a generally overcast sky is sometimes seen at sea in the daytime, not to be confused with the green coloration of part of the clear twilight sky in the west. This day-time coloration is associated with bad or windy weather, or is considered a prognostic of such weather. Observations of it and of the accompanying or subsequent weather will be welcomed, as it is not yet fully understood. It appears to occur most frequently in the Roaring Forties.

When clouds, particularly middle and upper clouds, occur about the time of sunset or sunrise, or in bright twilight, their coloration is often very beautiful. The cloud colours are mainly shades of orange, rose or red, since the direct sunlight illuminating the cloud has passed through a great length of the lower layers of the atmosphere. Shades of purple are sometimes seen, since a cloud may at the same time be indirectly illuminated by scattered blue light from higher atmospheric levels. On rare occasions colouring of exceptional magnificence occurs.

Colour phenomena also occur in a cloudless sky during the twilight periods. These vary considerably and are best developed in arid or semi-arid land regions. Some of those which occur more commonly everywhere are mentioned here. The Primary Twilight Arch appears after the sun has set, as a bright, but not very sharply defined segment of reddish, or yellowish light, resting on the western horizon. After the sun has set, a pink or purple glow may be seen, covering a considerable part of the western sky, known as the First Purple Light. It reaches its greatest brightness when the sun is about 4º below the horizon, and disappears when it is about 6º below.

At sunset, a steely-blue segment, darker than the rest of the sky, begins to rise from the eastern horizon. This is the shadow of the earth thrown by the sun on to the Earth' s atmosphere. The Earth-shadow is bordered by a narrow band of rose or purple colour, called the Counterglow. The whole rises fairly quickly in altitude, the shadow encroaching on the counterglow and soon obliterating it. With increasing general darkness the edge of the shadow weakens, but may sometimes be traced up to its passage through the zenith. In the later stages of twilight, this shadow edge has come down nearly to the western horizon, leaving a slightly more luminous segment between it and the horizon. This is the Secondary Twilight Arch. Just before the ending of astronomical twilight, it is sometimes seen as a fairly well defined whitish arch on the horizon, with an altitude of only a few degrees at its apex. This might be confused with an auroral arc visible at very low altitude.

Analogous phenomena, in the reverse order, occur before sunrise. Other colours are often seen in the cloudless twilight sky, portions of which may be green, yellow, orange or red, according to the amount of dust and water vapour present in the air. Instead of the purple light after sunset, the sky very often shows some shade of clear green, probably when the air is relatively free from dust.

Between the visible stars, brighter and fainter, the background of the clear night sky is not wholly dark. Part of the general luminosity of the sky is due to the accumulated light of the brighter telescopic stars, which cannot be seen as individual stars with the unaided eye. The remainder is due to the airglow, which may vary in intensity on different nights. The airglow is greenish in colour but it is usually too faint for the colour to be seen. In bright moonlight the sky is generally somewhat greenish, but it is probable that when the air is relatively dust-free and the full moon is at high altitude it becomes bluish. Opinion varies on this point, as the colour of faint light is not equally well seen by different persons.

Twilight is due to the illumination of the higher levels of the atmosphere by the sun when this is below the observer's horizon. The last stage of twilight is very faint and indefinite and it is not possible to say exactly when it ends. Astronomical twilight is defined as ending in the evening when the sun's centre is 18º below the horizon, since by that time sixth magnitude stars, the faintest that can be seen by the naked eye, have become visible in the region of the zenith.

Another and shorter twilight period, that of civil twilight, is recognized; this ends in the evening when the sun is 6º below the horizon. This is assumed to mark the ending of the time when outdoor labour is possible. The period of civil twilight is important to the seaman because experience has shown that subsequent to it the horizon is not sufficiently clearly visible to obtain good stellar observations. In the later stages of civil twilight such observations can be made, the brighter fixed stars being visible and the horizon still remaining clearly visible. Similar definitions apply to morning twilight.

The duration of twilight varies according to the latitude. It is shortest in the tropics where the apparent track of the sun down to the horizon is steepest. It also varies to some extent at different seasons, being shortest in all latitudes about the time of the equinoxes.

The following tables show the extent of these variations between the equator and latitude 60º N or S; AT and CT refer to astronomical and civil twilight respectively.

In the belt between latitude 481/2º N and the Arctic Circle there is no true night for some weeks of the midsummer period, as the sun does not sink as much as 18º below the horizon. There is a similar belt in the southern hemisphere, six months later, during the southern summer. In polar regions there is a long twilight period of about two months between the long polar periods of summer daylight and winter night.

At rare intervals abnormally long duration of twilight is observed. This is caused by the presence of fine dust suspended in the upper air. The dust may be due to a great volcanic eruption, such as that of Krakatoa in 1883 or to the fall of an exceptionally large meteor, such as that of 30 June 1908 in Siberia. Observations of exceptionally bright and long-continued twilight will be of value.

A waterspout is a whirlwind over the sea, appearing as a funnel-shaped column usually extending from the lower surface of cumulonimbus cloud to the sea. In travelling over the sea this column often becomes oblique or bent; it may become looped. The spout is in rapid rotation and the wind around it follows a circular path. Although very local, this wind is often violent, causing confused but not high sea. A noise of 'rushing wind' may be heard. In most cases, a waterspout forms downwards from the base of the cloud, appearing in its early stages as a dark funnel hanging from the cloud. The sea surface below becomes agitated and the funnel finally dips into the centre of the spray. The waterspout may last from a few minutes up to half an hour or more. Sometimes the spout, formed of condensed water vapour, does not reach the sea, and retreats up into the cloud. Several may be seen at the same time.

Formation. There are a number of theories which attempt to explain the formation of a waterspout. These theories may be classified into those which relate to the origin of the more severe tornado storm spout of the tropics and subtropics, and those which relate to the milder 'fair weather' spout of tropical and temperate latitudes.

The tornado spout. This may form over the sea but is more likely to have formed over land and subsequently to have passed out to sea. Its formation may result from the horizontal shear between warm and cold air currents existing up to considerable heights in the atmosphere. Such conditions normally occur along a cold front or cold occlusion surface. The tornado storm waterspout may damage even a large vessel if it passes directly over it, the damage being caused partly by tornadic winds, partly by the deluge of water sometimes released, and partly by the suddenly reduced pressure, although this effect has probably been exaggerated.

The fair weather waterspout. This type of waterspout is believed to be formed partly by convection processes. Under conditions of a high temperature lapse rate near the sea surface, a small parcel of warm air becomes a little warmer than its environment and begins to rise. Rotation is caused by the converging surface winds sucked in under the rising air parcel and energy gained by the atmospheric instability is augmented by the latent heat of condensation of the water vapour present. The initial convectional ascending air current may occur directly below cumulonimbus cloud in which case it may penetrate the cloud, and the rotation increases until a complete waterspout is formed. Although the 'fair weather' waterspout should cause no real damage to a larger vessel, it should be avoided by the small- boat mariner.

Though waterspouts are infrequent in high latitudes, their frequency does not depend wholly on latitude: indeed, a number have been reported from North Sea oil platforms. In general, more are observed in lower latitudes but their frequency in tropical and equatorial regions varies considerably in different oceans. Waterspouts are commonest in the following regions: Equatorial Atlantic, South Atlantic, eastern coast of the United States south of 35º N, Gulf of Mexico, eastern Mediterranean, Bay of Bengal and the Gulf of Thailand.

Sketches or photographs of waterspouts, and details of the mode of formation and dissipation, are of value. The diameter of the spout and the direction of rotation should be noted. If it is possible to determine the rate of rotation, this information is very valuable. Sometimes a streak or mark on the spout enables this to be done. The spout is a hollow tube; double-walled spouts have occasionally been recorded. The approximate vertical height of a spout may be found by vertical sextant angle, together with the estimated distance from ship. The height of a waterspout from sea surface to cloud base is usually from 1000 ft (300 m) to 2000 ft (600 m). It may, however, be as little as 100 ft (30 m) or as much as 5000 ft (1 500 m). There is a marked variation in observed diameters of waterspouts, from about 1 m up to about 200 m. The weather conditions, including instrumental readings, should be logged before, during and after the sighting of a waterspout; it is also helpful to know the position of the spout relative to the parent cloud and relative to any shower falling from it, e.g. whether the spout was at the leading edge or the rear of the cloud.