An arc of colour produced when sunlight is refracted and reflected inside falling raindrops. Seen only with the sun low and behind you and rain ahead of you.
PhenomenaWhat is a rainbow?
A rainbow is not an object in the sky and not a thing you can walk up to. It is an angle. Sunlight entering a falling raindrop is bent on the way in, reflected off the inside of the back surface, and bent again on the way out. The amount of bending depends slightly on wavelength, so the light leaves the drop spread into a spectrum, and it leaves concentrated at a particular angle from its original direction.
That angle is what you see. The primary bow appears as a circle of radius about 42 degrees centred on the antisolar point — the point directly opposite the sun, which is your own shadow’s head. Millions of drops each send you one colour, depending on exactly where they sit relative to that 42-degree cone. Red appears on the outer edge, violet on the inner.
This also means every person sees their own rainbow, formed by different drops. Two people standing a few metres apart are looking at slightly different bows, and a photograph records a third. There is no location, so there is nothing at the end of it.
The geometry, and why timing matters
Because the bow is centred on the antisolar point, and the antisolar point is as far below the horizon as the sun is above it, the height of the sun sets everything.
When the sun is high, the antisolar point is deep below the horizon and the whole 42-degree circle sits underground — no rainbow is visible from the ground at all. The sun must be below about 42 degrees of elevation. As the sun sinks towards the horizon, the bow rises; at sunrise or sunset it reaches its maximum, a full semicircle standing on the horizon.
This constraint bites harder in South Asia than in Europe. At the latitudes of Chennai, Kochi, Colombo or Karachi the midday sun climbs steeply and stays above 42 degrees for a long stretch of every day through much of the year. A rainbow over Kerala is almost necessarily a morning or late-afternoon phenomenon. Northern hill stations in winter, where the sun stays lower, keep the window open for longer.
The other rule follows from the same geometry: the sun must be behind you and the rain in front of you. Look east in the late afternoon, west in the early morning. During the monsoon, the most reliable moment is the half hour after a convective shower passes eastward while the western sky opens up.
Rainbows in South Asia
The best conditions are broken convective rain with clear sky behind it, which is exactly what South Asia produces in quantity:
- Late monsoon and post-monsoon afternoons across the Western Ghats, the Konkan and Malabar coasts, and inland Maharashtra and Karnataka, when afternoon cells pass through and the sky clears before sunset.
- The northeast and the Meghalaya hills, where cloud breaks over steep terrain give frequent bows and, from a ridge, occasionally a very complete one.
- Pre-monsoon Nor’wester evenings in West Bengal, Bihar and Bangladesh, when a squall clears rapidly and the low sun catches the trailing rain.
- Tamil Nadu and coastal Andhra Pradesh during the northeast monsoon, October to December, when showers are common and the sun sets early enough to help.
- Waterfall spray bows, which do not need rain at all. Jog Falls, Dudhsagar and Athirappilly all generate them on sunny days when you stand with the sun behind you.
Deserts and dry seasons produce very few: the Thar, interior Sindh and Balochistan simply lack the falling rain.
Other bows and related effects
Double rainbows. A second, fainter arc sometimes appears outside the first at a radius of about 51 degrees. It is made by light that reflected twice inside the drop, which reverses the colour order — red on the inside, violet on the outside. The band of noticeably darker sky between the two bows is real and has a name: Alexander’s dark band. No light is returned to your eye from that angular zone.
Fogbows are white. Fog droplets are too small for clean colour separation, so the arc appears as a pale band. They are seen in hill stations such as Shillong, Darjeeling, Munnar and Mussoorie when hill fog is lit from behind.
Moonbows use moonlight instead of sunlight. They need a bright moon near full, rain opposite it and a dark sky, and to the eye they look ghostly grey because the light is too faint for colour vision.
Full-circle rainbows are visible from an aircraft or occasionally from a high ridge, where there is nothing below the horizon to cut the circle off.
The rainbow in South Asian languages
Almost every South Asian language names the rainbow as a bow. Hindi has इंद्रधनुष (indradhanush), the bow of Indra, the Vedic deity of rain, storm and thunder — a name that links the arc directly to the weather that makes it. Bengali has রংধনু (rongdhonu), “colour bow”. Urdu uses قوس قزح (qaus-e-quzah). Tamil has வானவில் (vaanavil), “sky bow”.
Schoolchildren across India, Pakistan and Bangladesh learn the colour order by the mnemonic VIBGYOR — violet, indigo, blue, green, yellow, orange, red, read from the inside of the arc outward. The spectrum is in fact continuous; the count of seven is a convention that comes from Newton, not from the physics.
Frequently asked questions
Why can I never reach the end of a rainbow? Because it is an angle, not a place. The bow is defined relative to your own eye, so moving towards it moves it with you. There is no end and no fixed position on the ground.
Why are there no rainbows at midday? The bow is centred opposite the sun. With the sun high overhead, that centre is far below the horizon and the arc falls entirely below ground level. In much of South Asia this closes the window for several hours around noon for a large part of the year.
Why is the second rainbow’s colour order reversed? The secondary bow is made by light that reflected twice inside each drop. The extra reflection flips the geometry, so red ends up on the inner edge instead of the outer.
Do rainbows mean the rain is over? Not necessarily, but they do mean there is clear sky on the sun’s side. During monsoon convection the shower producing the bow is moving away from you, so locally the rain is usually easing.
Where can I check whether conditions suit a rainbow today? Look for a forecast with showers rather than continuous rain, plus sunshine. Mausam Online shows hourly rain probability and cloud cover for cities including Mumbai, Kochi, Shillong, Chennai, Darjeeling and Dhaka.