Scientific Attitude on Understanding Disappearance of Rainbow

By Daewon W. Byun and Seon Ki Park, April 23, 1999

Rainbow is often used as a metaphor for the optimistic view about life. It represents hope, harmony, childhood dreams, and utopia. Occasionally, rainbows are observed in double arcs, thus conveying friendship. Many commercial or public entities are embracing it to brighten the image of their companies or organizations. Names such as "Rainbow Reality", "Rainbow Optics", "Rainbow Coalition", "Rainbow Chapel" are very common.

Recently, there was a short article in The Korea Herald [March 20, 1999; and was reported in IEKAS 99-14, April 9, 1999] under the title, "Rainbow Disappearing from Seoul", describing that there hasn't been a rainbow in the skies of Seoul for more than three years since October 1995, according to the Korea Meteorological Administration (KMA). In this regard, the article displays a heavy euphemistic connotation. We understand the underlying reporter's concern about the well being of the Korean society. However, here, we would like to discuss a few scientific aspects of the cause of rainbow disappearance - a simple rebuttal to the theory forwarded by a professor in the same article.

The professor attributed it to the worsening air pollution causing the raindrops to become smaller. It is well known that the primary features of rainbows can be explained with Descartes' geometric optical theory. However, to describe the detailed variation of each rainbow as well as the supernumerary bows, Maxwell's wave theory for diffraction of light is required.

For the story of rainbow disappearance, we can focus our discussion on the primary or secondary bows, those that can be described by the Cartesian theory.

We see a rainbow in the sky only when, with the sun behind us, we look at a cloud from which rain is actually falling, and it is illuminated by the sunlight in certain positions. Rainbows are not usually observed on water-drop clouds themselves, unless rain is obviously falling from them. Raindrops, rather than the cloud droplets, normally form rainbows. To explain the disappearance of rainbows, we need to study at least the following two hypotheses;

In a nut shell, we cannot say simply we know the cause of rainbow disappearance. To study the cause of "rainbow disappearance", there needs to be a better understanding of many factors involved. The rainbow observation can not be explained with just simple conjecture. To investigate this problem, we may need to employ a very sophisticated cloud model that includes size distributions of nuclei and aerosol/pollution model resolving aerosol size distributions and species concentrations to examine the combined effects. Both of these must include detailed representations of radiation processes for entire solar spectrum. Further, the modeling results must be validated with careful observations revealing radiative characteristics of polluted atmosphere.

Social scientists are asked to explain current political and economic conditions. In many situations, they can forward their opinions with limited information and the practices are generally accepted because impromptu human behavior often determines the outcome of the situation. Similarly, natural scientists are asked to provide the so-called expert opinions on certain environmental phenomena or problems as they become more serious and affect people's well being--either direct, indirect or metaphoric perception. In contrast to questions on social issues, questions related to natural phenomena do not allow much excuse to use simple conjectures. We must strive to gather enough information and to study the issues more prudently before we can offer any answer to such questions.

Drawing the linkage between these issues, we ourselves may have committed some mistakes. The only consolation for us is that here we try to address the issue of attitude of scientists, one of social issues which do not seem to have absolute answer, unlike to the natural science questions as the disappearance of rainbow.

Corresponding authors:

Daewon W. Byun*, Physical Scientist

Seon Ki Park#, Research Scientist