halo; unknown radius (photo anonymous)
Rainbows and glories are optical phenomena which can only appear if drops of water are
present in the atmosphere (or on the ground). All these phenomena become impossible when
the drops freeze, forming snow- or ice-crystals. But transparent ice-crystals will also refract
and reflect sunlight, as a result of which various circles, spots or arcs appear in the sky, and a
number of these sometimes display beautiful rainbow-like colours. Such circles usually have
the sun (or moon) as a central point, as the rainbow always has the anti-solar point as a
central point. Optical phenomena caused by refractions in ice-crystals usually appear on the
same side of the sky as the sun. Collectively, they are called haloes. They are not at all
scarce: a halo can be observed on an average of two out of three days somewhere in the
Netherlands. But in contrast to the more exceptional rainbow relatively few people have seen
them. The rainbow, however, appears in the darker section of the sky opposite to the sun,
whereas most haloes are visible at a relatively short distance from the sun. For this reason,
their intensity of light is so high that it is often difficult to look into them with the naked eye. But
with sunglasses these beautiful phenomena can be easily observed, frequently when there are
cirrus-like clouds near the sun.
There exists a great variety of types of halo, because ice-crystals can refract or reflect sunlight
in many different ways, but the most important and frequently occurring haloes can be divided
roughly into three groups:
- Light-intensive coloured
phenomena at about 22º from the sun (or from the moon). The
form can be a circle with the sun at its centre (the 22º halo), but it can also be very bright
spots on either side of the sun (the parhelia) or curved arcs of various shapes above or
below it. Red is nearest the sun and usually very conspicuous. Orange and yellow are
usually still distinctly visible, but the other colours are paler. This category of halo occurs
most frequently.
- Fainter but more colourful
phenomena of the same character and with the same
sequence of colours at about 46º from the sun. Their abundance of colours may exceed
that of rainbows.
- Uncoloured (white) spots
and bands in several places in the sky. Here haloes belonging to
this group will be called reflection haloes.
circumzenithal arc (photo by Capt. B.C.W. Norton)
By moonlight nearly all haloes look colourless, because under those conditions our eyes are
almost insensitive to colours. It is only the bright parhelion (at night, called paraselene or
mock-moon) that still appears to display colours. Figure 1 is a sketch of the most important
halo phenomena. The haloes are certainly worthy of observation, not only for their beautiful
colours and variety but also because they may alternately appear or disappear very quickly.
Some sub-horizon haloes have also been included in fig. 1; generally, they are only visible
from aeroplanes or from high mountains. The most frequent varieties are the 22ºhalo (c), the
parhelia (mock-suns) at 22º on either side of the sun (a), the upper and lower tangent are to
the 22º halo (d and e), the circumzenithal arc at 46º above the sun (b) and the sun pillar (i).
The 46º halo (f), the parhelic circle (g) and the paranthelia (h) are much more uncommon.
Special attention should be paid to the lower tangent arc to the 22º halo (fig. 1, c), because at
solar elevations between 10º and 13º it suddenly transforms its shape into a very remarkable
loop. The intersection of the loop is white and it is always exactly in the position where the
sub-sun
Fig. 1 Sketch of the most important haloes: parhelion (a), circumzenithal arc (b), 22º halo (c), upper
and lower tangent arc (d and e), part of the 46º halo (f), parhelic circle (g), paranthelion (h) and the
sun pillar (i), The subsun (j) and subparhelion (k) are haloes below the horizon, Haloes (g), (h), (i)
and (j) are uncoloured.
should appear. At these solar elevations the arc changes its outward appearance so quickly
that one runs the risk of being taken by surprise; it transforms its shape from minute to
minute. Being, however, completely below the horizon, the are cannot be seen from the
ground, but only from an aeroplane or from a high mountain. Also, the very rare lower tangent
arc to the 46º halo behaves in exactly the same way, at solar elevations between 22º and 28º,
by turning into a loop with an intersection in the position of the sub-sun. I do not know if it has
ever been observed.
