A rainbow is a common atmospheric optical phenomenon that occurs when sunlight refracts through water droplets in the atmosphere, creating a multicolored arc in the sky. This beautiful spectacle involves several complex processes including reflection, refraction and dispersion of light each occurring at different angles, thus separating the constituents of light spectrum into distinct colors. Despite its arresting beauty, a rainbow is essentially an optical illusion; it does not physically occupy a specific position in the sky.
Color Order in a Rainbow
Rainbows can be seen around fog, sea spray, or waterfalls, and the colors on a primary rainbow always appear in order of their wavelength, from longest to shortest. The sequence stars with red, followed by orange, yellow, green, blue, indigo, and finally violet. Rainbows are the result of two main phenomena – refraction and reflection. Here, a refracted wave appears “bent”, while a reflected wave seems to “bounce back” from a surface or other wavefront.
Rainbow Occurrence and Climate Change
Recent research links the frequency of rainbow occurrence to climate change, painting a transformative picture of future patterns of rainfall and cloud cover. Human activities, predominantly burning fossil fuels, are warming the atmosphere, subsequently altering the volume and patterns of rainfall and cloud cover. These changes will disturb the distribution of rainbow occurrence. An increase in greenhouse gas emissions leads to changes in cloud cover and liquid precipitation, which are projected to cause a net surge in global rainbow days annually.
Global Rainbow Projections by 2100
Based on current trends and predictions, the mean annual global rainbow days are estimated to rise by 4.0–4.9% by the end of this century. However, not all regions will experience an increase. Around 21–34% of land areas will lose rainbow days. These areas are projected to have lower total precipitation by 2100, excluding regions in Central Africa, Madagascar and central South America – all of which are predicted to witness more annual dry days and less total annual cloud cover.
On the other hand, approximately 66–79% of regions will observe an increase in rainbow days under higher emission futures. Countries like India, Mali, Niger, Chad, Sudan, and Ethiopia are among those which will see a rise in the number of rainbow days. Rainbow gain hotspots are mostly situated at higher latitudes and very high elevations such as the Tibetan Plateau, where anticipated warming is expected to result in less snow and more rain.
Rainbow Distribution Patterns
Interestingly, two rainbow gain hotspots, namely eastern Borneo and northern Japan, will see an overall increase in precipitation but more dry days annually. Climate change alters the evaporation and convergence of moisture, leading to changes in precipitation and cloud cover patterns, thereby impacting the distribution of rainbow occurrence.
Interpreting Rainbow Phenomenon – A UPSC Civil Services Examination Question
In the UPSC Civil Services Examination of 2013, a question concerning the formation of rainbows was posed. The inquiry sought answers on the physical phenomena responsible for the creation of rainbows, i.e., dispersion, refraction, and internal reflection. A rainbow is essentially a meteorological phenomenon caused by these three phenomena working in synergy with water droplets resulting in a spectrum of light appearing in the sky. Consequently, all three – dispersion, refraction, and internal reflection – rightly explain the cause behind rainbow formation.
