“At CEPAL, we are dedicated to the scientific study
of optical atmospheric phenomena.”
Classification of Rainbows
Jean Louis Ricard1,2, Peter Adams2 and Jean Barckicke2,3
1 = CNRM, Météo-France,42 Avenue Gaspard Coriolis, 31057 Toulouse, France
2 = CEPAL, 148 Himley Road, Dudley, West Midlands DY1 2QH United Kingdom
3 = DPrévi/Compas,Météo-France,42 Avenue Gaspard Coriolis, 31057 Toulouse, France
[email protected]
Summary
Rainbows are the most beautiful and most spectacular optical atmospheric phenomenon. Humphreys (1964) pointedly noted that “the “explanations” generally given of the rainbow [ in textbooks]
may well be said to explain beautifully that which does not occur, and to leave unexplained which does” . . . “The records of close observations of rainbows soon show that not even the colors are
always the same”.
Textbooks stress that the main factor affecting the aspect of the rainbow is the radius of the water droplets. In his well-known textbook entitled “the nature of light & colour in the open air”, Minnaert
(1954) gives the chief features of the rainbow depending on the diameter of the drops producing it.
For this study, we have gathered hundreds of pictures of primary bows. We sort out the pictures into classes. The classes are defined in a such way that rainbows belonging to the same class look
similar. Our results are surprising and do not confirm Minnaert's classification. In practice, the size of the water droplets is only a minor factor controlling the overall aspect of the rainbow. The main
factor appears to be the height of the sun above the horizon. At sunset, the width of the red band increases, while the width of the other bands of colours decreases. The orange, the violet, the blue and
the green bands disappear completely in this order. At the end, the primary bow is mainly red and slightly yellow.
What is a rainbow?
A rainbow is an arch of light exhibiting the spectrum colours in their order, caused by drops of water falling through the air. It is seen usually in the sky opposite to the sun at the close of a shower and
also in the spray of waterfalls. In the brightest or primary bow, often the only one seen, the colours are arranged with the red outside. Above the perfect bow is a secondary bow, in which
the colours are arranged in reverse order; this bow is dimmer, because of a double reflection within the drops. When the sunlight enters a raindrop it is refracted, or bent, by and reflected from the drop
in such a way that the light appears as a spectrum of colours. The colours can be seen, however, only when the angle of reflection between the sun, the drop of water, and the observer's line of vision is
between 40° and 42°.When the sun is low in the sky the rainbow appears relatively high; as the sun rises higher, the rainbow appears lower in the sky, maintaining the critical 40°- to 42°-angle. When
the sun is more than 42° above the horizon no rainbow can be seen because the required angle passes over the head of the observer. The Alexander band is a dark band lying between the primary
and the secondary bows . The supernumerary arcs are extra bands, usually pale pink or green in colour,. They are often seen on the inside of the primary rainbow. They result from the
diffraction and the interferences of light rays which emerge from water droplets in the same direction. While the primary rainbow colours result from a concentration of the different coloured rays in
the maximum deviation direction, there are many other paths for light through the droplets, so that at angles below the rainbow angle there could be rays of different wavelengths which emerge
together. The number of visible supernumerary arcs depends upon the size of the raindrops.
Previous classification: Minnaert (1954)
Class
Diameter
MIN_1
Member
Class
Diameter
1-2 mm
MIN_4
0.08 – 0.10 mm
MIN_2
0.5 mm
MIN_5
0.06mm
MIN_3
0.2 - 0.3 mm
MIN_6
< 0.05mm
1
Member
“At CEPAL, we are dedicated to the scientific study
of optical atmospheric phenomena.”
Classification based on the solar height
Description of the class
the blue and the green bands are very the yellow band is the most intense.
vivid. The orange and red bands are The orange and the red bands are
barely visible.
intense. The blue and the violet bands
are less intense.
Solar Height
70°
the blue and the green, blue and
violet bands are barely visible.The
red, orange and yellow bands are
vivid.
the blue and the violet bands have
completely vanished. The red band is
largely dominant. A hint of green,
yellow and orange bands are
noticeable. The supernumerary bows
are red
88.3°
89°
85.5°
CEPAL Model of Rainbow
Member of the class
New classification of rainbows
Class
Definition
RAB_1
Member
Class
Definition
ACV
SAB
SUP
RAB_5
RAB_2
ACV
SAB
no SUP
RAB_3
RAB_4
Member
Class
Definition
No violet OR
no Blue
RAB_9
Only Blue and
Red
RAB_6
No Green
RAB_10
Only Yellow
and
Orange/Red
ACV
WAB
SUP
RAB_7
No Violet
AND No Blue
RAB_11
Only Red
ACV
WAB
no SUP
RAB_8
No Blue AND
No Green
RAB_12
Other
The new classification is a solid basis for challenging our understanding of rainbows.
1) for each class, can we explain the observed variability ?
2) Rainbows at sunset/sunrise (RAB_10 and RAB_11)
3) Supernumerary area (RAB_1 to RAB_4)
Member
4) Influence of the temperature of the water droplets
cknowledgment
This research was supported by the CEPAL (Centre d'Etudes des Phénomènes Atmosphériques Lumineux), the CNRM (Centre National de Recherches Météorologiques), and Météo-France.
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