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;
If KMA's observation is accurate, the question becomes if air pollution reduces the size of raindrops and thus there is little chance that lights are reflected and refracted by the raindrops. Heavy pollution, in particular with high concentration of airborne fine particles, can influence the condensation process in the atmosphere. There could be some relation between cloud droplet size and availability of condensation nuclei as well as hygroscopic particles such as sulfate aerosols. In fact, if more fine aerosols exist, they have more chances to serve as condensation nuclei thus make more cloud droplets. Then, there are many complex mechanisms to make these cloud droplets to grow to large raindrops. The two prominent mechanisms are auto-conversion (from cloud to rain), which is a statistical coalescence process, and collection (through collision and sweeping up). Of course these processes are strongly related to terminal velocity and vertical motions (updraft and downdraft) inside the cloud. Thus the growth of raindrop is highly dependent upon microphysical processes and dynamical behavior inside the cloud. The convective motion inside severe storms also plays a vital role in making large frozen raindrops grow to big hailstones.
As stated before, rainbows are usually associated with the in-air precipitation, i.e. raindrops that stays long enough in the air-not so large to fall right away but large enough the geometric optical phenomena to happen. In many occasions, these raindrops evaporate before they reach the ground. It is difficult to see connection between the "rain drop" size and air pollution. We think how big the raindrops can be is totally dependent on microphysical and dynamical features in the cloud, not directly to air quality. If there are urbanization effects on precipitation, some papers suggest enhanced convective activities thus increasing precipitation. Therefore the possibility of this hypothesis being correct is remote.
To answer this question, we need to have more precise observation for the rainbow. When lights are reflected and refracted by small waterdroplets, white bows or rainbows with very dull colors appear. It is called fog-bows. Let's assume KMA's observation does not classify them as rainbows. One important factor for the "rainbow" observation is the visual range between the observer and where the raindrops remain in the air (as well as the relative position of the sun with respect to raindrops and observer). There is some possibility that, depending on the size distribution of air pollution particulates and compositions, the attenuation of different sections of sun's spectrum (i.e., rainbow colors and ultraviolet and infrared frequency ranges) can be different. Attenuation of light is caused by the absorption and scattering (Mie and Rayleigh) by the particles and air molecules. Particles, especially black carbons, are known to absorb visible range spectrum mostly. This light-absorbing substances in the atmosphere can be responsible for the brown appearance of urban haze and the discoloration of the sky. Is the sky in Seoul so much loaded with this type of particles to blur the rainbow colors? With a simple statement as reported in the article, it is impossible to know if this hypothesis is correct, although it is a possibility.
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.
Daewon W. Byun*, Physical Scientist