Yesterday, April 4, 2012 a hola caused some excitement among residents of Antigua and Barbuda. The halo became very visible at around mid-day, prompting at least a dozen calls to the Met Office from curious onlookers wandering what was the cause of it and if it was a portent of hazardous weather. Some very religious persons may have even thought that it was a sign from God.
A halo is a ring of light encircling and extending outward from the sun or moon. Such a display is produced when sunlight or moon light is refracted as it passes through ice crystals. Hence, the presence of a halo only indicates that cirriform clouds (cirrus and cirrostratus) are present in the sky as these clouds are made up of virtually 100% ice crystals. These clouds generally form at above 25,000 feet (five miles) in the atmosphere, where the temperature is well below the freezing point. Often times, halos go unnoticed as they are not as brilliant as the one seen on Wednesday.
A halo is usually seen as a bright, white ring, but there are refraction effects that can cause it to have colours like the one seen yesterday. Halos come in two sizes; the more common 22 degrees, which was seen yesterday and the less common 46 degrees. To understand the cause of the colours of a halo, we must first examine white light and refraction more closely.
White light is the name given to what the human eye sees when all the colours that make up the visible light spectrum are combined; the visible light spectrum is made up of the colours of the rainbow and these colours combined make white lighting or simply “light” or “visible light. This is the same light that makes it possible to view one’s surroundings. When white light passes through a glass prism, it is refracted and split into a spectrum of visible colours. In other words, when shined through a prism, white light is broken into the separate colours of the visible light spectrum. Each wavelength of light is slowed by the glass prism (ice crystals) but each is slowed a little differently. Because longer wavelengths (red) slow the least and shorter wavelengths (violet) slow the most, red light bends the least, and violet light bends the most. The breaking up of the white light by “selective” refraction is called dispersion. As light passes through ice crystals of cirriform clouds, the ice crystals act like glass prisms resulting in dispersion of white light into the colours of a rainbow. Because of the different wavelengths, dispersion causes red light to be on the inside of the halo and blue light on the outside.
Halos occur very frequently in our part of the world. They are not a portent of anything really but they could precede or come after bad weather, which normally produce cirriform clouds. It is a bit surprising that many persons saw one for the first time yesterday. I guess, for many of us, we don’t look up unless it is raining or a plane is passing overhead.
So the next time you see a halo, know that there are cirriform clouds (high clouds) present causing white light to separate into its various colours.