|Home | Welcome | What's New | Site Map | Glossary | Weather Doctor Amazon Store | Book Store | Accolades | Email Us|
Weather Almanac for April 2005
In this poem, Longfellow refers to one of the longest standing and generally correct in the mid-latitudes pieces of weather lore: the appearance of a halo around the sun or moon as a harbinger for coming storm and precipitation. Richard Inwards classic collection Weather Lore (1893) lists no less than thirty weather rules, sayings, or proverbs pertaining to halos.
The facts behind this piece of lore tell us that very often in the cloud decks around a storm system, an extensive band of cirrostratus clouds lies ahead of an approaching warm front. In the majority of cases, when sun- or moonlight passes through these ice clouds, a halo forms around it. In the typical textbook storm and front sequence, cirrostratus follows forerunner cirrus clouds and precedes the lowering of the cloud deck until precipitation develops in the nimbostratus near the warm front. In other circumstances, cirriform-clouds may develop in the anvil of approaching thunderstorms and sport a foretelling halo.
Halos can also often be seen downwind of the summit ridge of a high mountain range through orographic ice clouds those clouds formed when moist air pushes high over the mountains. A fourth category of halos develops in light show showers and even in clear frigid air. For this reason, the polar regions often produce the world's most impressive displays of halos. The best are seen in the Antarctic interior where the ice crystals tend to pure and homogenous in shape and size (see South Pole halo display below).
What Is A Halo?
By broad definition, a halo is a group of optical phenomena, in the form of rings, arcs, pillars, or bright spots around the sun or moon, produced by the refraction or reflection of light by ice crystals suspended in the atmosphere (The National Snow and Ice Data Center, Boulder, CO).
This definition includes ice crystal optical phenomena that I have written about elsewhere (light pillars, and sun and moon dogs) and some very interesting but infrequently seen phenomena such as Parry arcs, anthelion and a variety of tangent arcs. The word halo conjures up the image of a circle, but the majority of halo phenomena are not seen as complete circles, appearing rather as arcs with enough curvature to suggest a circle fragment.
Here, I intend to narrow my discussion to the two most common halos, the 22-degree and the 46-degree halos. Halos arise in all seasons when cirriform clouds are present, but the winter months often offer the best conditions for seeing halos as frontal storms pass along the southward-dipping polar front and frigid air provides a favourable environment for ice crystal formation at all altitudes. In warmer periods of the year, high altitude cirrus clouds may form vast ice-crystal sheets through which the sun or moon may shine and form halos.
The Crystal Base
Halos and related phenomena are produced by the refraction, spectral splitting and, at times, internal reflections of light by ice crystals suspended in the atmosphere. Ice crystals form in a variety of hexagonal shapes depending on air temperature and humidity, but those crystals responsible for halos are usually elongated columns and flat plates. In crystal terminology, the hexagonal face of the crystal is called the basal facet and the six rectangular faces connecting the two basal facets are called the prism facets. Typically, the larger crystal dimension is 0.05 to 0.1 mm across. The column and plate crystal shapes are similar except for their relative prism facet depth which is long in columns and thin in plates. If the ratio of the height to the basal facet radius is less than two, the crystal is considered a plate, otherwise it is a column. You can think of a plate as a squished column or a column as a drawn-out plate.
The hexagonal-plate crystal patterns (right) are flat and hexagonal with the basal facet width much larger than the prism facet depth, resembling microscopic stop signs or dinner plates. The formation of plate crystals is favoured at air temperatures from 0°C to -4°C (32°F to 25°F) and from -10oC to -20oC (14oF to -4oF). When their size is very small, crystal plates tumble randomly through the air as they fall. When larger plate-shaped ice crystals fall unimpaired, drag forces orient them horizontally so that their larger, basal facet surface parallels the earth, descending like a large maple leaf drifting down from a tree.
When plates and/or column ice crystals are suspended in the air, bright light passing through them will be refracted on entrance to the crystal and again on exit. These refracted light rays split into a spectrum of colours with red-orange on the inside and blues in the outer band. The most dominate colours in a halo are usually the longer wavelengths: red, orange and yellow. There are several reasons for this including a tighter angle of spread for longer wavelengths and more scattering and absorption of the shorter, blue wavelengths. Often, the bluish sky background masks the shorter wavelengths. In many circumstances, the mixing and dispersion of colours from many crystals produce a whitish halo brighter than the surrounding sky.
While typical crystal dimensions are 0.05 to 0.1mm, the optimum size for halo production appears to be around 0.05 to 0.2 mm for the larger facet. When crystals are smaller than 0.01 mm, halos are weak and diffuse. Crystals larger than 0.05 mm refract and reflect light cleanly to produce distinct halos. The sharpest halos occur when the crystals have precisely aligned crystals larger than 0.1 mm.
Regardless of their proportions (lengths as well as facet ratios), all crystals have identical angles among the facets which produce regular and predictable halos. The three angles are 60o, 90o and 120o, the last angle playing the least significant role in halo phenomena. (While the angles making up the hexagonal facet are 120o each, the passage of light through a hexagonal crystal acts as if it has passed through a 60o triangular prism.) As a result, researchers studying the halo family classify halos as either 60o or 90o events.
In passing through two faces of the hexagonal facet, the light path deviates from the straight line path by 22o. Should the light ray pass through two faces whose angle forms 90o, the deviation from the straight line is 46o. These are the two most common paths and form the two most common halos: the 22-degree halo and the 46-degree halo.
In a cirrus cloud, aerodynamic forces orient the falling ice crystals to some degree of regularity based on their shape. Plates generally fall like leaves with their broads facet facing up/down, but may tumble around their horizontally-aligned axis. Column crystals also often fall with their long axis in the horizontal and may rotate around that axis or around their vertical axis like a helicopter's propeller blade. Halo phenomena arising from spinning crystals produce the most complex patterns because their presentation to the sun/moon light changes greatly.
When there is a range of crystal sizes and shapes with a mix of stable orientations, random orientations and spinning crystals, many different halo phenomena may be presented simultaneously. Such halo displays are often observed in the optimum conditions of the polar regions such as the one below from the South Pole.
Halo Display at the South Pole
To Purchase Notecard,
Now Available! Order Today!
The BC Weather Book: