The anatomy of a rainbow

The photograph that took a mile

Galen Rowell's Rainbow over the Potala Palace has the sort of alignment that makes you wonder how long someone waited for it. A bright band of colour arrives at a palace, the building catches the sun, and the mountains behind it sit in darkness. Everything seems to have found its place.

In October 1981, in Lhasa, Tibet, Rowell saw the rainbow and ran nearly a mile to put it where he wanted it in the frame. Dick Dorworth, who was there, confirms the story in his recollection of Rowell. The weather supplied the light. Rowell supplied the viewpoint.

That is the part I wanted to explore in the next Nigel North film. We spend a lot of time deciding when to take a photograph. A rainbow makes the question of where to stand wonderfully literal.

Nigel goes back to 1981

In Physics Says You Cannot Steal a Rainbow. I Went Back to 1981 to Try Anyway., Nigel takes a film camera, an outfit for the era, and one fairly substantial cheat: a phone with Inverza on it. He has five minutes of screen time to learn what Rowell understood, cope with the altitude, and negotiate with a dog.

It follows the same idea as our journey to Ansel Adams' Moonrise: use a little fictional time travel to look closely at a real photograph and the science behind it. The character and historical scenes are made with AI. Rowell's photograph and his achievement are real; you can explore his work through Mountain Light.

Watch Nigel North's rainbow episode · 5 min 15 sec. If the player does not load, watch it on YouTube.

What happens inside a raindrop

Sunlight bends as it enters a raindrop. Some of that light reflects from the back of the drop, then bends again on its way out. Water bends the different wavelengths by different amounts, spreading white sunlight into colours. For the primary rainbow, the route involves two refractions and one internal reflection. The Royal Meteorological Society explains that journey in more detail.

The light leaving many drops is concentrated around particular viewing angles. The red outer edge of the primary bow sits roughly 42° from the direction directly opposite the sun; violet is nearer 40°. That direction is called the antisolar point. If your shadow is visible, the shadow of your head points towards it. The National Weather Service's rainbow guide explains the colour order and the geometry.

Picture a cone with its tip at your eye and its axis pointing away from the sun. Raindrops along that cone send the rainbow light you see. As you move, a different set of drops supplies it. Steven Businger's study of rainbow science and photography describes this observer-centred geometry. It explains why a rainbow has no fixed landing place in the field, and why two people standing apart see slightly different bows.

For composition, that is useful. Moving sideways changes the palace's direction from your camera while the bow remains tied to the sun's direction. You can bring the two together, provided there is still sunlit rain in the right part of the view. Rowell's run was a photographic decision with an optical explanation.

The useful number is 42°

The sun's height tells you approximately how high the primary bow can reach. When the sun is 10° above the horizontal, the antisolar point is 10° below it. Add the bow's roughly 42° radius and its red outer edge tops out around 32° above the horizontal.

A lower sun gives you a taller arch. As the sun approaches the horizon, the bow approaches a semicircle above a level horizon. Above about 42° sun elevation, the primary bow lies below the horizontal, so it disappears from an ordinary ground-level view. From a high viewpoint, with sunlit droplets below you, you can see more of the circle, sometimes all of it. These are the same viewing angles, with more of the scene available beneath your feet. See the NWS low-sun explanation and Businger's full-circle examples.

This gives you a useful first check for a location: a low sun behind you, rain in front of you, and sunlight reaching that rain. You can be standing in shade and still see a rainbow. The droplets are the part that needs direct light.

The second bow and the dark band

Sometimes a fainter bow appears outside the first. Its light has reflected twice inside the drops, and its colours run the other way: red on the inside, violet on the outside. It sits roughly 50–53° from the antisolar point.

The space between the two bows often looks darker. This is Alexander's dark band: the primary and secondary rainbow paths send much less light back towards you at those intervening angles. The sky and clouds still contribute light, so the band is darker rather than black. Roland Stull's chapter on atmospheric optics walks through both effects.

That darker sky can do a lot for a photograph. Try giving the second bow room in the frame before zooming in on the brightest part. Then look at the foreground: a small patch of sunlit grass or stone may be enough to connect all that sky to a place.

Planning your own rainbow photograph

The useful forecast is a combination of things: showers, a break for the sun, and a subject in the right direction. Evening rain to your east with a low western sun is a classic arrangement. Morning reverses it. Rainbows can happen with ordinary showers too; a thunderstorm is not required.

Inverza helps put those pieces together. Horizon Paths shows the sun's direction and elevation against the terrain at your chosen spot. The weather forecast adds the likely rain and cloud pattern. In relevant storm guidance, AI Chat can also point out possible rainbow geometry, including a low-sun time and the direction opposite it. That is a candidate to investigate: the app cannot know the exact position of every shower or promise when a rainbow will appear.

1. Choose a subject and leave room to move. Save a spot with a view towards the expected rain. Look for a few possible camera positions around your building, tree or mountain.
2. Check the sun in Horizon Paths. Find a time when it is low behind you. Use 42° minus its elevation to estimate the top of the bow, then think about how much sky the frame needs.
3. Ask about the overlap. Try AI Chat with: “Could showers and low sunlight overlap here this afternoon, and which direction should I watch for a rainbow?” Read the answer alongside the forecast, then check current radar and the actual sky as the time approaches.
4. Make the alignment on location. When a bow appears, take a frame, then move if a better relationship with the foreground is possible. Check the bright colours and sunlit surfaces for clipped highlights. A wider photograph first gives you something to keep while you work on a tighter composition.

If thunder is audible, take shelter in a substantial building or an enclosed hard-topped vehicle. A rainbow can appear while lightning is still a hazard; the National Weather Service's lightning guidance applies even when your part of the sky has cleared.

This is what interests me about Rowell's photograph, and why it belongs in a film about planning. Understanding the light gave him a reason to move. His eye told him when to stop.

Try it with a familiar place next time showers are forecast. Check the sun, look towards the rain, and see what a few steps do to the picture. Nigel's trip to 1981 is there if you would like a dog and a little time travel with the explanation.