What Is Blue Hour, and How Long Do You Actually Get?

There is no shortage of good writing about how to shoot blue hour. White balance, exposure blending, why city lights and sky finally balance — the photography publications cover all of it well, and this is not another one of those.

What none of them tell you is how long it lasts where you are. That number varies by a factor of three across the populated world, and it decides whether blue hour is a comfortable working window or a scramble.

The definition

Blue hour runs while the sun sits between −4 and −6 degrees below the horizon, in the evening. It picks up exactly where golden hour ends and finishes at the end of civil twilight.

The colour comes from what is left in the sky when direct sunlight has gone. Longer wavelengths have been scattered out of the light reaching you along its very long path through the atmosphere; what remains is dominated by blue, and by ozone absorption in the Chappuis bands, which removes some of the yellow and orange and deepens the blue further.

Two degrees of sun altitude is a narrow band. That is why blue hour is short, and why the name is even less accurate than golden hour's.

The numbers, plainly

At Mumbai or Nairobi, blue hour is around eight to ten minutes. If you are setting up a tripod, checking a composition and bracketing exposures, that is one attempt. There is no time to walk to a second viewpoint.

At London, it is twelve to sixteen minutes depending on season. Enough for two setups if they are close together.

At Stockholm, it runs to about half an hour near the solstices. This is why northern European cities have a reputation for blue-hour cityscapes: not because the light is different, but because there is enough of it to work in.

Above the Arctic Circle in summer the sun never reaches −6 degrees, so civil twilight never ends and blue hour never closes. The table says "never ends", which is literally what happens.

Blue hour is not "after sunset"

A common instruction is to wait twenty or thirty minutes after sunset. That works at mid-latitudes and fails elsewhere.

Blue hour starts when the sun reaches −4 degrees. In Nairobi that is about fourteen minutes after sunset. In Stockholm in June it is over an hour. Using a fixed offset means arriving late in one place and standing around in the cold in the other.

The gap between sunset and the start of blue hour is roughly two-thirds of the sunset-to-dark interval, which is tabulated for these same cities.

The morning version is not a mirror

Morning blue hour runs from −6 up to −4 as the sun rises, and it is close to the same length as the evening one on the same date. The difference is what surrounds it.

In the evening you arrive in daylight, watch the light fall, and your eyes adapt gradually. In the morning you arrive in the dark, and the sky is already changing before you have finished setting up. The window is the same size; you have less warning.

Practically, the argument for morning is that the air is usually cleaner and cities are empty. The argument against is that you must be in position before it begins, and the beginning is defined by an angle you cannot see coming.

What actually helps

Get the number for your city before you go. Every city page here shows tonight's blue hour window and the full month. If it says twelve minutes, plan one composition.

Start before it starts. The transition through golden hour into blue hour is continuous; there is no moment where the light becomes correct. Shooting from the end of golden hour through the end of blue hour gives you a sequence to choose from rather than a single guess.

Watch the weather, not the clock. A high overcast kills blue hour by removing the graduated sky that produces the colour. Broken high cloud with a clear western horizon is the setup that produces the best of it — the same conditions that make a good sunset, described in more detail in our piece on clouds.

In the far north, reconsider the plan. If the table says the window never ends, you do not have a blue hour to catch. What you have instead is several hours of low, even, cool light, which is a different and often better problem.

Why the blue is that particular blue

Two mechanisms produce the colour, and they are not the same one that makes the daytime sky blue.

The first is ordinary Rayleigh scattering, which after sunset is illuminating the upper atmosphere from below and to the side rather than from overhead. The light reaching your eye has taken a very long path and has lost most of its red end.

The second is ozone. There is a layer of it around 20 to 30 kilometres up, and it absorbs in the orange and yellow — the Chappuis absorption bands. During the day this is swamped by direct sunlight. After sunset, when the light you are seeing has travelled a long horizontal path through that layer, the absorption becomes visible and pushes the sky further towards blue than scattering alone would.

This is why blue hour looks deeper and more saturated than a daytime sky, and why the effect is strongest when the air is clean. Haze adds forward-scattered white light and washes it out.

Planning across a trip

If you are travelling, the window changes underneath you.

Fly from Mumbai to Stockholm in June and blue hour goes from about nine minutes to about thirty. That sounds like a gift, and it is, but it changes how you work: in Mumbai you pick one composition and commit, in Stockholm you can move between two or three viewpoints in the same window.

Travel in the other direction and the mistake is worse, because the habit you have built does not warn you. Photographers who learn blue hour in northern Europe and then work in the tropics routinely miss it, because they are still walking to their position when it has already ended.

The rule that travels well: look up the number for the city you are in tonight, not the one you learned in. Every city page here shows it.