22° Halo Visible around the Sun
On March 26th, 2012 sky watchers were pleasantly surprised by a circular rainbow visible around the sun. This phenomenon is called a 22° Halo, one of the many optical phenomena, that forms a 22° circle around the sun. To check this stretch out the fingers of your hand at arms length. The tips of the thumb and the little finger then subtend around 20°. Place the thumb over the sun and the halo will extend a little from the little finger tip. The halo is always the same diameter, regardless of its position in the sky. This phenomenon is occasionally also visible around the moon and artificial light in very cold weather. A Halo is formed by refracted sunlight in hexagonal ice crystals in the higher atmosphere. This phenomenon is not related in any way to solar storms, i.e. a massive burst of solar particles that often interact with the Earth’s magnetic field causing disturbances in telecommunication.
Figure 1: The Halo visible for sky watchers on March 26th, 2012. Photo courtesy of Otniel D'Aguiar.
Halos, with the 22° halo being no exception, appear when the sky is covered by higher clouds that are made up out of ice crystals. Clouds are made out of water droplets. As the height increases, the temperature decreases up to a point where the water droplets freeze and become ice crystals. These higher altitude clouds are called cirrus and are located 5 km and higher. A halo forms when the sky contains millions of variously oriented ice crystals. When these ice crystals align perpendicular to the sunlight as seen by an observer, the observer can see a halo.
The physical term for this phenomenon is called refraction, i.e. bending of the sunlight. White light is made up of different wavelengths. Each wavelength has a different color and a different refraction (bending) index. As the white sunlight passes through the hexagonal ice crystals it is refracted twice, upon entrance and upon exit, and produces deviation angles between 22 and 50° . Since red light has a smaller refraction angle, and blue a larger angle, inner edge of the circle will be reddish, while the outer edge is bluish. As no light is refracted at the smaller angels than 22° the sky is darker inside the halo.
Many people associate the appearance of a halo with rain, and its presence was used in ancient times to predict the weather, before meteorology was developed. This association is due to the fact that halos are often seen on the advancing edge of a frontal system (i.e. mid latitude weather systems). However many halos are not associated with these weather systems and hence seeing one is not necessarily a sign of wet conditions.
The ingredients for the formation of halos are hazed skies due to thin cirrus clouds and bright sunlight. Unfortunately these atmospheric displays cannot be predicted the way astronomers can pinpoint the dates and times of meteor showers and eclipses. While the specific weather conditions offer a chance, it is simply a question of luck.
