What is blue hour?

What is blue hour?

Blue hour is a name photographers, painters, and atmospheric scientists give to the short period of twilight, immediately before sunrise and immediately after sunset, when the dominant tint of the sky is a deep saturated blue rather than the warm reds and yellows of the moments either side of it. Wikidata, the structured-data sister of Wikipedia, registers the term as a multilingual concept — French l'heure bleue, German Blaue Stunde, and equivalents in dozens of other languages — capturing how widely the same observation has been named across cultures.⁶

The window itself sits inside civil twilight — the phase of dusk or dawn during which the Sun's centre lies between the horizon and 6° below it.³ Within that phase, the Sun spends a couple of minutes at an altitude where the sky is still bright enough for unaided activity, but the warm sunset reds in the western sky have given way to a uniform deep blue overhead. The site's solar tools draw the upper boundary of blue hour at 4° below the horizon, so blue hour spans solar depressions of 4° to 6° on this site. Photographic references such as PhotoPills and SunCalc give a similar range; some widen the band to 8°. There is no standards-body figure to anchor it.

The result, in everyday terms, is the brief sky-darkening transition between an orange sunset and ordinary night — or, in the morning, between night and a yellow sunrise. The Sun is below the horizon, but the upper layers of the atmosphere are still lit by direct sunlight bending around the curvature of the Earth, and the sky brightness produced by that indirect lighting is still significant. The colour is a property of the indirect light: long atmospheric paths, plus a specific absorption feature in atmospheric ozone, leave blue dominant.¹

Why is the sky blue during blue hour?

Two atmospheric effects combine to produce the blue tint, and isolating either one in turn is the easiest way to see how they work together.

The first is Rayleigh scattering: the elastic scattering of sunlight by gas molecules small compared with the wavelength of visible light. The rate at which a gas molecule scatters a photon scales as the inverse fourth power of the photon's wavelength, so blue light at around 450 nanometres is scattered roughly six to seven times more strongly than red light at around 700 nanometres. NASA Space Place gives the plain-language version: "blue light is scattered more than the other colors because it travels as shorter, smaller waves."⁷ Through the daytime sky, this is what makes the daylight sky overhead look blue. After sunset, when the only sunlight reaching the lower atmosphere is the indirect glow of the upper atmosphere, the residual sky brightness is still dominated by the same scattered-blue contribution.

The second is the Chappuis absorption of atmospheric ozone, named for the French chemist Jules Chappuis, who in 1880 noticed that light passing through ozone gas takes on a blue tint and traced the colour to absorption bands in the visible spectrum.⁵ Ozone has two broad absorption peaks in the visible range, centred near 575 and 603 nanometres — the yellow-orange portion of the spectrum. The bands are weak compared with ozone's strong ultraviolet absorption, but the path length through the stratosphere becomes very long when the Sun is low, and the cumulative effect on the residual yellow-and-red component of the sky's illumination is significant. The result is a selective subtraction of warm wavelengths from the upper-atmospheric sunlight that would otherwise contribute to twilight skyglow, leaving blue more dominant than Rayleigh scattering alone would predict.

Lee, Meyer, and Hoeppe make the modern attribution explicit. Their 2011 measurement campaign at the Antarctic Georg von Neumayer Station found that "zenith skylight is often distinctly blue during clear civil twilights, and much of this color is due to preferential absorption at longer wavelengths by ozone's Chappuis bands."¹ The same paper notes that aerosol scattering and absorption matter at least as much as ozone for the off-zenith colours of twilight, and that the contributions are difficult to separate cleanly even with controlled measurements — a useful caveat against over-confident single-mechanism claims about the blue-hour phenomenon.

The relative-air-mass figures that drive both effects are extreme at twilight angles. The Kasten and Young 1989 formula — the standard reference for atmospheric optical path lengths — gives an air mass of 1 at the zenith, about 5.6 at 10° above the horizon, and approximately 38 at the geometric horizon.⁸ During blue hour the direct beam from the Sun grazes through tens of atmospheres' worth of air at altitudes still high enough to be lit, and it is the spectrum of that light, after long-path scattering and ozone absorption have worked on it, that the upper atmosphere reradiates downward toward the observer.

How long does blue hour last?

The duration of blue hour at any given location and date is set by how long the Sun takes to descend through the 4° to 6° depression band. That depends on the angle the Sun's apparent path makes with the horizon, which in turn depends on the observer's latitude and the Sun's declination on the date.

Near the equator, the Sun's path is nearly perpendicular to the horizon, so it cuts through the band quickly: blue hour at the equator is often only ten minutes long, and the sky moves from warm sunset to dark night within twenty or twenty-five minutes total. At mid-latitudes the path is more slanted; the same band takes fifteen to thirty minutes around the equinoxes, with longer windows near the local summer solstice when the Sun rises and sets at the steepest angle to the horizon for the date. At high latitudes the path is shallow enough that the Sun can take an hour or more to drop from 4° to 6° below, and around the local summer solstice the Sun may never reach 6° below the horizon at all — in which case civil twilight, and with it blue hour, persists from sunset until sunrise.²

Wikipedia summarises the dependence as a working range of "20–96 minutes" depending on date and place, with the longer end belonging to high-latitude summer.² The figures should be read against the same caveat as for any photometric twilight definition: the boundaries are conventions, and a few-minute shift in either threshold changes the duration noticeably. The shape of the dependence is fixed; the exact number of minutes is a function of which thresholds the source has chosen.

Why is blue hour important in photography?

Blue hour is the photographic counterpart of golden hour, with three properties that combine to give it its own visual character.

The first is the colour temperature. Direct sunlight at midday measures around 5,500 K and reads as effectively white. The light reaching the ground during blue hour is the indirect glow of the upper atmosphere, with its yellow and red component partially absorbed by ozone and its blue component preferentially preserved by Rayleigh scattering. The effective colour temperature climbs into the 9,000 K to 12,000 K range, well into the cool-blue side of the colour gauge. The light is unmistakably cool against the eye and against any artificial illumination — sodium street lamps, tungsten interior lights, neon signage — that the photographer chooses to include in the same frame.

The second is the contrast. The Sun is below the horizon and casts no direct shadows. The dominant illumination is the diffuse glow of the sky, which lights the scene from above and from all directions in roughly equal measure. Faces, buildings, and landscape contours are lit softly, with shadows that have no hard edge and no clear directionality. Architectural and cityscape photographers exploit the property: a building lit by interior light or by streetlamps will appear set against a sky that retains a strong, deep, photographically saturated blue, with the warm interior light glowing against a cool exterior in the same exposure.

The third is the brightness balance with artificial light. Around the moment the Sun is at 5° to 6° below the horizon, the sky brightness has fallen far enough that interior tungsten lamps, street lamps, neon signs, and the lit faces of buildings reach roughly the same exposure value as the sky itself. Photographers can capture both the lit sky and the lit interior in the same exposure without the sky burning out as it would at full daylight or going completely black as it would deep into night. The window in which the balance holds is short — five to fifteen minutes around the centre of blue hour at most mid-latitude locations — which is why photographers and city film crews schedule the shoots tightly around the published times.

How is blue hour calculated?

Calculating blue-hour times for a location and date is a Sun-position problem, exactly like sunrise, sunset, and the three twilight phases. The algorithm computes the Sun's altitude as a function of time, latitude, longitude, and Earth's orbital geometry, then finds the moments at which the altitude crosses the −4° (blue-hour upper bound) and −6° (blue-hour lower bound, the start of civil twilight) thresholds in each direction. The standard implementation is the U.S. National Oceanic and Atmospheric Administration's solar calculator, accurate "to within a minute for locations between +/- 72° latitude" and valid from the year −2000 to +3000 — the same engine the site uses for every Sun event it computes.⁹

The four blue-hour times for a single day are:

• Morning blue-hour start — the moment the Sun reaches 6° below the horizon on its way up, i.e. the start of morning civil twilight.³
• Morning blue-hour end — the moment the Sun reaches 4° below on its way up.
• Evening blue-hour start — the moment the Sun descends through 4° below.
• Evening blue-hour end — the moment the Sun reaches 6° below the horizon on its way down, i.e. the end of evening civil twilight.³

Both thresholds are pure geometric definitions referring to the position of the Sun's centre. They include no atmospheric-refraction correction; refraction is folded into the separate angle used for sunrise and sunset (about 0.833° below the horizon, accounting for both standard refraction and the Sun's apparent radius), and is not part of the twilight or blue-hour definitions.³⁹

The site's golden hour calculator reports the four daily blue-hour times alongside the four golden-hour times, and the twilight calculator publishes the civil-twilight bounds within which blue hour occurs. The two together cover the full sequence from late afternoon through the end of civil twilight at sunset, and from the start of civil twilight at dawn through to mid-morning.

How does blue hour relate to twilight and golden hour?

Blue hour, golden hour, and the three civil/nautical/astronomical phases of twilight are different ways of slicing the same continuous interval — the gradient of falling light from broad daylight through to dark night, or the same gradient running in reverse at dawn. Each window is bounded by a particular solar altitude, and the windows tile the gradient without overlap.

The full evening sequence at a mid-latitude location runs:

• Golden hour — Sun above the horizon, between roughly +6° and 0° altitude, casting the warm, low-angle direct light prized in photography.
• Sunrise / sunset — Sun's upper edge on the horizon (Sun's centre at about 0.833° below).³
• Blue hour — Sun between 4° and 6° below the horizon (a subset of civil twilight); the dominant sky tint shifts from warm to deep blue as Rayleigh scattering and ozone Chappuis absorption combine on the long atmospheric path.¹
• Civil twilight — Sun between 0° and 6° below; horizon and brightest objects still visible without artificial light.³
• Nautical twilight — Sun between 6° and 12° below; horizon distinguishable at sea against a dark sky.³
• Astronomical twilight — Sun between 12° and 18° below; faint stars start to show but residual scattered sunlight is still measurable.³
• Night — Sun more than 18° below; scattered solar contribution drops below natural starlight.³

Morning runs the same sequence in reverse. Blue hour is therefore the brief, photographically-named subset of civil twilight, immediately deeper than golden hour, and the section of the gradient where the sky is most strongly tinted by ozone Chappuis absorption.¹

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