Weather Phenomena, 30 March 1980
In the May 1980 OASI Newsletter, Alan Smith reported a sighting of a Moon halo as follows:
On Sunday 30 March 1980, at 20:45 UT, several members of OASI saw a very rare phenomenon caused by the light from a full Moon shining through high level cirrus clouds. The first phenomenon noticeable was a 22.5° halo around the Moon together with several rays that appeared to emanate from the Moon itself. The halo was yellow/orange in colour; the inner edge had a red tint and the outer edge a green tint. One of the rays struck a cloud and caused a full rainbow-type halo, again at 22.5°. At first the halo appeared rather dim, but by 21:00 UT it had brightened considerably over about 15° of its circumference and, by using averted vision, it could be traced around a full circle. Over the bright portion of the circumference, the halo exhibited an orange colour, the inner edge continued to exhibit a red tint and the outer edge exhibited a grey tint.
Moon halo, 30 March 1980. (A J Smith.)
The Sun halo and Moon halo are fairly common spectacles. They are visible when light from the Sun or Moon shines through ice crystals associated with a thin, whitish veil of cirrostratus cloud. The halo appears as a faint ring around the Sun or Moon and can vary in size and colour. The space inside the ring is generally very faint white. If the halo is strongly defined, a red tint may be visible on its inner edge and a yellow tint on its outer. Although most haloes are centred on the Sun or Moon, this is not always the case as some are centred on the zenith.
In fact, Alan's observations cover just one example of an interesting weather phenomena and there are many other that you might look out for. This article describes the most interesting examples.
As is widely known, the rainbow is associated with showery weather and always appears in the opposite point of the sky to the Sun. Under ideal conditions it is possible to see two bows: the larger one is the secondary and the smaller the primary. The colours occur in the opposite order in the two bows. A particularly brilliant rainbow can exhibit up to seven distinguishable colours, in the following order from the outer band inwards:
- Primary: red, orange, yellow, green, blue, indigo, violet.
- Secondary: violet, indigo, blue, green, yellow, orange and red.
From the ground it is possible to observe only the half circle of a bow, but from an aeroplane at a sufficient height it is possible to observe the complete circle.
When a rainbow appears after the first shower during a spell of fine weather, it is usually a sign of an impending change to more unsettled conditions. During an unsettled spell, a rainbow appearing in the evening denotes an improvement in coming weather conditions.
Some sky spectacles are caused by the Sun's rays falling on dust particles and air molecules in the atmosphere. One such phenomenon is the sun pillar, caused by the reflection of the Sun's rays on the vertical sides of columnar crystals in the atmosphere. (A Moon pillar can occur under similar conditions.) The sun pillar appears as a column of either red or white light extending vertically above and below the Sun. Sometimes, the vertical column is crossed by a similar horizontal bar. A complete cross formed in this manner is called a heavenly cross. The sun pillar is rarely seen in the British Isles and the heavenly cross is even more rare.
During a showery spell, lines of watery-looking light radiating from the Sun are sometimes observed. This is sometimes referred to as the Sun drawing water; the meteorological term is crepuscular rays.
Another sky phenomenon is known as the green ray. It appears as a brilliant emerald colouration on the very last edge of a setting Sun on the very first edge of a rising Sun.
Considerable quantities of dust in the air, such as may occur after a large volcanic eruption, will often cause strange sky spectacles. A blue Sun was reported from many parts of the world after the great eruption of Krakatoa in 1883. The Sun appeared during the midday hours shining a deep azure blue; at sunrise it was a bright light blue, and at sunset a dark blue. The eruption had carried particles of dust to a great height in the upper atmosphere and created a dense haze which resulted in the unusual colours. A blue Sun has also been reported for two or three consecutive days after some great dust storms in the Sahara and other extensive desert areas. Sometimes dust in the atmosphere can result in the Sun appearing yellow: over Cairo during a great dust storm, the Sun appeared a pale yellow colour while the sky immediately around it appeared pale blue and, at night, the Moon also appeared pale yellow, surrounded by a similar blue tint.
Among other types of unusual weather phenomena are coloured rain and snow. Rain has been known to fall as red or blue, as it did for example during March 1935 in the Shetland Isles after a heavy thunder storm; descriptions spoke of it looking very much like blue-black ink diluted with water. The accepted explanation was localised pollution and atmospheric conditions. There have also been many reports of coloured rain falling in Italy. Red rain has also been reported falling in New Zealand: one such occasion was on 26 October 1929, following two days of a curious smoky haze over South Island. The haze disappeared after the storm of red rain. The only time red rain has fallen over England was on 21 and 23 February 1903 when it was believed to be caused by dust raised into the atmosphere by a great storm over the Sahara and carried northwards to Europe by the winds. Coloured snow has been reported from various parts of the world, usually red or green and sometimes yellow or brown. The colours are caused by minute vegetable organisms known as Protococcus Nivalis.
One phenomenon which is astronomical, but can be mistaken for an atmospheric feature, is the Gegenschein or counterglow. This is a very faint glow visible on the ecliptic, opposite the position of the Sun, only slightly lighter than the general illumination in the sky; it is visible sometimes, on clear moonless nights, from dark-sky locations. Spectral analysis reveals it to be sunlight scattered by minute dust particles in the interplanetary medium in the inner solar system.