Why Sunsets Are Red and Orange, and How Long the Colour Lasts
The physics is in every explainer. The part nobody adds is that how long it lasts depends entirely on where you are standing.
Sunlight arriving at the top of the atmosphere is white. What reaches your eye at sunset is red and orange, and the reason is that the atmosphere has removed the rest of it along the way.
That much is well explained in a dozen places. What none of them add is the consequence: how long you get to watch it depends on your latitude, and the difference runs to a factor of three.
The scattering
Air molecules scatter light, and they scatter short wavelengths far more strongly than long ones. The relationship goes as the inverse fourth power of wavelength, which is steep: blue light at 450 nm is scattered roughly four times as strongly as red at 650 nm.
This is Rayleigh scattering, and it explains both the blue sky and the red sunset with one mechanism.
Overhead, in daylight, sunlight passes through a relatively short column of air. Some blue is scattered out of the direct beam and bounced around the sky in all directions, which is what you see when you look away from the sun. The sky is blue because that is the light being scattered towards you from everywhere.
At sunset, the sun's light enters at a shallow angle and travels through a far longer path — roughly 38 times the vertical thickness of the atmosphere when the sun is on the horizon. Over that distance nearly all the blue and green have been scattered out of the beam before it reaches you. What survives is the long-wavelength end: orange and red.
The same light, the same mechanism, opposite results, purely because of path length.
| City | Latitude | Today | 21 June | 21 December |
|---|---|---|---|---|
| Longyearbyen | 78.2°N | all night | all night | — |
| Tromsø | 69.6°N | 2h 50m | all night | — |
| Reykjavík | 64.1°N | 2h 03m | all night | — |
| Stockholm | 59.3°N | 102 min | 2h 48m | 2h 48m |
| London | 51.5°N | 81 min | 102 min | 102 min |
| New York City | 40.7°N | 65 min | 75 min | 75 min |
| Cairo | 30.1°N | 57 min | 63 min | 63 min |
| Mumbai | 19.1°N | 52 min | 56 min | 56 min |
| Bogotá | 4.6°N | 49 min | 53 min | 53 min |
| Nairobi | 1.3°S | 49 min | 52 min | 52 min |
| Rio de Janeiro | 22.9°S | 53 min | 58 min | 58 min |
| Sydney | 33.9°S | 59 min | 66 min | 66 min |
| Wellington | 41.3°S | 66 min | 76 min | 76 min |
| Ushuaia | 54.8°S | 88 min | 119 min | 119 min |
What the table measures
The colour is not an event, it is a process that runs while the sun is low. The table above measures the window from +6 degrees to −6 degrees of sun altitude — from the point where the light has clearly warmed to the end of civil twilight, after which there is not enough light left to be red.
That window is 27 minutes in Nairobi and well over an hour in Stockholm in summer. Same physics; different geometry.
The reason is the one that governs everything else on this site. Near the equator the sun's path meets the horizon nearly perpendicular, so it crosses those twelve degrees of altitude quickly. At high latitude it meets the horizon at a shallow angle and takes far longer to cover the same ground.
If you have ever felt that tropical sunsets are abrupt and northern ones linger, that is not an impression. It is the table.
Why some are better than others
Path length explains the colour. It does not explain why one evening is spectacular and the next is not, and the answer to that is what the light lands on.
A clear sky gives you a warm gradient near the horizon and little else. The dramatic sunsets happen when there is high cloud to catch the reddened light from below, while the low western horizon stays clear so the light can get out to reach it. That combination, and how to spot it in a forecast, is the subject of its own article.
Aerosols matter too, and not in the way people assume. Fine particles high in the atmosphere — volcanic sulphate, transported dust, wildfire smoke aloft — enhance the effect and produce the most vivid sunsets on record. The eruption of Krakatoa in 1883 produced skies vivid enough that fire brigades in several countries were called out to non-existent fires.
Larger particles near the ground do the opposite. Urban haze and pollution scatter across all wavelengths rather than selectively, adding white and washing the colour out. The idea that pollution makes better sunsets is mostly wrong: it makes redder suns and duller skies.
The same mechanism explains why the sun itself looks dimmer and more orange the lower it gets, and why it is safe to look at for a moment near the horizon when it is not at any other time. The atmosphere is removing a very large fraction of the total energy, not just the blue.
Why the best colour comes after the sun has gone
The peak is usually a few minutes after sunset, not at it.
Once the sun is below the horizon, the light illuminating any cloud above you has travelled an even longer path and is redder still. At the same time the ground and lower air are already in shadow, so the contrast between the lit cloud and everything else is at its strongest.
This is the single most common mistake people make: packing up when the sun disappears. The colour window in the table above extends well past sunset, and its most saturated part is inside that extension.
The morning version
Sunrise works identically — the same path length, the same scattering, the same colours. Any difference is in the air, not the physics.
Mornings often have cleaner air, because overnight cooling settles particulates and there has been no day of traffic and industry yet. That is the strongest argument in the sunrise-versus-sunset debate and it is a real one, though it varies enormously by place and season.
Against that, afternoon convection often builds the cloud that makes a sunset dramatic, and mornings can be flatly clear. Neither wins reliably.
Getting the window for your own city
Every city page here gives sunset, the golden hour window, blue hour and the end of civil twilight, which between them bracket the colour window described above. The monthly table shows how the whole sequence shifts day by day.
The practical version, if you want one line: arrive while the sun is still up, stay until civil twilight ends, and expect the best of it about five minutes after the sun has visibly gone.