Rainbows Haloes and Coronas
By Graeme McLeod
At meetings of the Association when members are showing photographs, the origin of some of the interesting things seen in the atmosphere comes up.
Rainbows are one of them. They happen when light from the Sun, behind the observer, are refracted and reflected by raindrops in front of the observer. The result is that the observer sees an arc, or with luck, a complete semi-circle showing the colours in the Sun's spectrum, with blue on the inside and red on the outside. The circle is always the same angular size, 42° in radius, whether the rain drops are far or near. The same happens in the spray from a waterfall, when the droplets may be only a few feet away. In a double rainbow, the radius of the outer rainbow is 52° , and the colours are the opposite way round because of second reflection in the droplets.
Sundogs and haloes around the Sun are produced by refraction only through ice crystals in the higher atmosphere. They are usually found at about 22° from the Sun but see below). The usual hand-span method can be used to check this. This time however, the colours are reversed with the red on the inside of the halo. (Beware of the power of the Sun!)
Coronas, (rings) much closer to the Sun or Moon can apparently be caused in two ways. Firstly by refraction through water droplets in clouds (Alto- cumulus). The colours will be as with the halo, with the red on the inside. They can also be caused by diffraction round ice crystals in the higher atmosphere. Here the colours will be the opposite way round with the blue on the inside and red on the outside. The rings will only be a degree or so across and vary with the phase of the moon. As the Sun and the Moon have an appreciable angular diameter (about 30' of arc, usually the inside is not seen and only the outside is coloured. (Again, beware of the Sun).
The presence of haloes and coronas can give some indication of changes of weather, perhaps the onset of rain. Lockhart suggests that haloes of radius 46° indicate a storm 24 hours hence. None of the other references however suggest the possibility of this radius of halo so it must be something of a rarity and worth looking out for.
Happy Halo Hunting!
Brief Explanation of terms.
Refraction. Light can change direction going into or coming out of a different substance through which it can pass. The amount of the change depends only on the properties of the substance.
Diffraction. This is the bending of light round an obstacle and depends on the size of the obstacle and the wavelength of the light. Red light bends more than blue. The rings here happen when light waves bending round opposite sides of an obstacle (ice crystals) come together making the waves bigger and hence the light brighter, always at particular points.
References
- Light:-R. Ditchburn
- Skywatching:- D. Levy
- The Subtlety of Rainbows - J. Hardwick in 'Physics World' Feb. 2004.
- The Weather Guide :- A.G. Fordyke
- The Weather Companion:- G Lockhart.