The Blue Hour: auto (top) and daylight white-balance

Since there is often much confusion about the origin of The Blue Hour, I offer this little essay as an attempt to explain what is going on. How am I qualified to do this? I'm an astrophysicist and a visual neuroscientist. I have studied extensively the properties of the atmosphere of our planet Earth and also, in preparation for future astronomical observations with the next generation of large space and groundbased telescopes, the likely properties of earth-like exoplanets. I am also looking at the visual adaptations made by animals that spend large amounts of their time hunting or being hunted in the deep blue of the twilit environment.


The Blue Hour
An explanation of the nature and physical cause of the phenomenon
by Robert (Bob) Fosbury

The Blue Hour (l'heure bleue) is the period of twilight starting when the Sun dips to a few degrees below the horizon and ending as the sky darkens towards night at the end of astronomical twilight (and in the reverse sequence at sunrise). The light from the sky reaching us and our surroundings during this period, especially during the central part called nautical twilight, is extremely blue; much bluer than a typical clear blue sky during daylight.

This phenomenon occurs under both clear and cloudy conditions since the physical process that causes it is quite distinct from that which causes the daytime blue sky (Rayleigh scattering). Indeed, the twilight blue has a distinctly different hue since it contains also a strong deep-red component that is near the limit of our visual sensitivity and therefore not vary apparent to our eyes. This particular hue is well represented by the the artist's pigment 'ultramarine', the famously expensive pigment derived from the lapis lazuli imported 'across the seas' from the ancient mines in Afghanistan. See the famous painting by Titian in the National Gallery: www.nationalgallery.org.uk/paintings/titian-bacchus-and-a...

The twilight blue results from the strong absorption of yellow, orange and red light during the long passage of sunlight as it passes almost horizontally through the atmosphere near sunset and sunrise. It works out that, on the horizon, the sunlight has to traverse around forty times as much atmosphere to reach us as it does if it is overhead at noon in the tropics.

The blue-coloured culprit for this theft of the orange is ozone ( www.bipm.org/en/bipm/chemistry/gas-metrology/ozone/ ), a fragile and very active molecule consisting of three, rather than the more normal two, oxygen atoms and occupying a layer in the atmosphere between altitudes of about fifteen and forty kilometres. This is the gas that protects life on Earth from the devastating effect that ultraviolet radiation would have on us if this component of sunlight were to reach ground level. While the orange absorption — resulting from what we call the 'Ozone Chappuis band' — is very much weaker than the protective ultraviolet absorption, the long atmospheric path at twilight strengthens its effect enough for it to have a profound effect on colour. The visual effect of this removal of much of the yellow, orange and red light is to leave the blue light to travel through a more transparent atmosphere to reach our eyes from the whole sky.

A good question to ask at this point would be: "OK, you say that ozone removes the orange light when the sun is on the horizon, But the sun is a bright orange at that time! — so what is going on?" Well, yes, that is a good question. To answer it we have to understand the meaning of two of the words that appear in my explanation above: scattering and absorption.

Scattering, as in 'Rayleigh scattering', is when a photon of light approaches an air (mostly oxygen or nitrogen) molecule in the atmosphere where the interaction between the two can bounce the light into a different direction without any change of colour. This is the way that photons from the sun can become part of the blue sky rather than reach us as direct sunlight. Scattering steals part of the sunlight and makes it into the blue sky. The light is changed in direction but not lost. The physics tells us that this kind of scattering is more likely to happen for blue light than for red: hence the sky blue. The sun looks orange/red on the horizon because a lot of the blue light has been removed from the solar image in your eye and been given to the blue sky. Rest assured however that the image of the orange sun would be a lot brighter if the ozone layer was removed!

The scattered skylight will then have its orange further diminished by ozone and so make The Blue Hour even bluer. Unlike the scattering process, when ozone absorbs orange light it swallows it and does not give anything back except, perhaps, a bit of warmth.

So, in summary, the blue hour blue is different from the blue sky blue and it is a direct result of the action of the ozone layer on incoming sunlight. The eye adapts itself to this blue and tends to make the twilight look visually grey. But the camera does not adapt, unless you have it set to "automatic white-balance" in which case try setting it to "daylight" instead. Then your Blue Hour photos will really look blue!


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