Reply by George Anderson, Photo Editor
1
Weather – March 2009, Vol. 64, No. 3 80 Readers’ forum Rainbow arc on the horizon Figure 1 shows a photograph taken at 1315 UTC on 1 April 2008, looking approximately north-east from near Haywood, Doncaster, South Yorkshire. There were showers of rain and sleet in the area, falling from isolated, shallow cumulonimbus in a strong wind. The view shows the horizon lit up with the colours of the rainbow, apparently with a shallow rainbow arc just above the level of the horizon. This was visible for only a few minutes before dispersing. There were no other superior arcs visible in connection with the one shown. My original conclusion was that, owing to the relatively low angle of elevation of the spring sun, a rainbow arc would be shallow and could be the cause of the phenomenon observed. However, about an hour later, another shower gave a more normal rainbow well above the horizon in a similar direction, causing me to discard this explanation. What was the reason for this arc being so shallow and is it unusual? Andrew K. Overton Doncaster, South Yorkshire DOI: 10.1002/wea.274 Reply by George Anderson, Photo Editor The answer to Andrew Overton’s question does indeed relate, as suggested, to the elevation of the sun. It is, however, a high elevation sun which results in a shallow rainbow near the horizon. Rainbow arcs are always centred on a point relative to the observer directly opposite the sun, known as the anti-solar point. The primary bow of a rainbow is essentially circle with a radius of 42° for the outer, red, part of the bow, centred on the anti-solar point. The radius of the inner, violet, part of the bow is slightly less. How Figure 1. ‘Rainbow arc’ on horizon, looking approximately north-east at 1315 UTC on 1 April 2008 from near Haywood, Doncaster, South Yorkshire. (© Andrew K. Overton.) Readers are invited to contribute short questions on any meteorological topic. We will endeavour to obtain answers to all submitted questions. much of a rainbow is seen by the observer depends on where the rain is actually falling (sometimes only part of a bow is visible) and also on the elevation of the sun above the horizon. At sunrise or sunset, when the sun is on the horizon, the anti-solar point is on the opposite horizon, and the primary bow is potentially (assuming a wide enough distribution of raindrops) a full semicircle. In this case, the rainbow arches in the sky as a semicircle to an elevation of around 40°. Usually, of course, the sun will be above the horizon by some amount and so an arc amounting to less than a full semicircle is normally seen. Interestingly though, from a high vantage point, such as on a mountain or from an aircraft, observers can sometimes see rainbow arcs that form a larger part of a circle because the local horizon is effectively lowered. As the sun rises in the sky, the corres- ponding anti-solar point lowers below the horizon and so the rainbow lowers in the sky. With the sun at an elevation approach- ing 40°, the anti-solar point is the same declination below the horizon and so the arc of the primary rainbow (with a radius of 42°) will be virtually on the horizon with only the top of the bow visible. If the sun rises to higher than 42° then the primary rainbow will be below the horizon. In the summer, when the sun is relatively high in the sky, rainbows are best seen with showers in the early morning or the late afternoon when the elevation of the sun is low. The same discussion above applies to a secondary bow, but in this case the angle involved is about 51°. At Haywood, near Doncaster in South Yorkshire, on 1 April at 1315 UTC the sun was at an elevation above the horizon of about 39°, hence only a small part of the primary rainbow arc was seen near the horizon. The top, red portion of the bow would have been about 3° above the horizon and the violet part of the bow a mere 1° above the horizon. An hour later, the sun had lowered in the sky by around 4 to 5°, and so the second rainbow arc witnessed was correspondingly about 4 to 5° higher in the sky, and thus appeared more normal. George Anderson Wokingham, Berkshire DOI: 10.1002/wea.307
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Transcript of Reply by George Anderson, Photo Editor
Wea
ther
– M
arch
2009
, Vol
. 64,
No.
3
80
Readers’ forum
Rainbow arc on the horizon
Figure 1 shows a photograph taken at 1315 UTC on 1 April 2008, looking approximately north-east from near Haywood, Doncaster, South Yorkshire. There were showers of rain and sleet in the area, falling from isolated, shallow cumulonimbus in a strong wind. The view shows the horizon lit up with the colours of the rainbow, apparently with a shallow rainbow arc just above the level of the horizon. This was visible for only a few minutes before dispersing. There were no other superior arcs visible in connection with the one shown.
My original conclusion was that, owing to the relatively low angle of elevation of the spring sun, a rainbow arc would be shallow and could be the cause of the phenomenon observed. However, about an hour later, another shower gave a more normal rainbow well above the horizon in a similar direction, causing me to discard this explanation.
What was the reason for this arc being so shallow and is it unusual?
Andrew K. OvertonDoncaster,
South YorkshireDOI: 10.1002/wea.274
Reply by George Anderson, Photo EditorThe answer to Andrew Overton’s question does indeed relate, as suggested, to the elevation of the sun. It is, however, a high elevation sun which results in a shallow rainbow near the horizon.
Rainbow arcs are always centred on a point relative to the observer directly opposite the sun, known as the anti-solar point. The primary bow of a rainbow is essentially circle with a radius of 42° for the outer, red, part of the bow, centred on the anti-solar point. The radius of the inner, violet, part of the bow is slightly less. How
Figure 1. ‘Rainbow arc’ on horizon, looking approximately north-east at 1315 UTC on 1 April 2008 from near Haywood, Doncaster, South Yorkshire. (© Andrew K. Overton.)
Readers are invited to contribute short questions on any meteorological topic. We will endeavour to obtain answers to all submitted questions.
much of a rainbow is seen by the observer depends on where the rain is actually falling (sometimes only part of a bow is visible) and also on the elevation of the sun above the horizon. At sunrise or sunset, when the sun is on the horizon, the anti-solar point is on the opposite horizon, and the primary bow is potentially (assuming a wide enough distribution of raindrops) a full semicircle. In this case, the rainbow arches in the sky as a semicircle to an elevation of around 40°. Usually, of course, the sun will be above the horizon by some amount and so an arc amounting to less than a full semicircle is normally seen. Interestingly though, from a high vantage point, such as on a mountain or from an aircraft, observers can sometimes see rainbow arcs that form a larger part of a circle because the local horizon is effectively lowered.
As the sun rises in the sky, the corres-ponding anti-solar point lowers below the horizon and so the rainbow lowers in the sky. With the sun at an elevation approach-ing 40°, the anti-solar point is the same dec lination below the horizon and so the arc of the primary rainbow (with a radius of 42°) will be virtually on the horizon with only
the top of the bow visible. If the sun rises to higher than 42° then the primary rainbow will be below the horizon. In the summer, when the sun is relatively high in the sky, rainbows are best seen with showers in the early morning or the late afternoon when the elevation of the sun is low. The same discussion above applies to a secondary bow, but in this case the angle involved is about 51°.
At Haywood, near Doncaster in South Yorkshire, on 1 April at 1315 UTC the sun was at an elevation above the horizon of about 39°, hence only a small part of the primary rainbow arc was seen near the horizon. The top, red portion of the bow would have been about 3° above the horizon and the violet part of the bow a mere 1° above the horizon. An hour later, the sun had lowered in the sky by around 4 to 5°, and so the second rainbow arc witnessed was correspondingly about 4 to 5° higher in the sky, and thus appeared more normal.
George AndersonWokingham,
BerkshireDOI: 10.1002/wea.307
