The Blue Hour

Just before sunrise and just after sunset, the parts of the sky high above the oranges and reds on the horizon can appear a blue that is deeper and more intense than any during the day. This time is known as ‘the blue hour’, l‘heure bleue in French, and it has a colour that is profoundly calming. The blue can also add an intensity to layers of cloud like these on the flanks of a distant Cumulonimbus spotted by Douglas Dubler over West Neck Beach, Long Island, US.

Closely linked to the blue hour is the more familiar ‘golden hour’. This is when the Sun is just above the horizon and its rays shine with warm, orangey tones. This effect results from the way the low atmosphere scatters away the bluer end of the visible light spectrum. Travelling at a shallow angle through the dense lower atmosphere, the light of the golden hour arrives only once the scattering effects of its long journey have removed all but the warmer colours of the spectrum.

The deep sapphire of l’heure bleue is also the result of scattering, but this time with an added boost courtesy of the ozone layer. The light reaching you from the upper sky, whether it is filtered through cloud or not, shines not from the Sun directly, since that is below the horizon. Instead, it is scattered towards you by the gasses of the atmosphere. And these are better at scattering bluer light. But the depth of the blue around this time comes from the way ozone in the Stratosphere absorbs light. The gas tends to soak up light in the middle of the visible spectrum – the greens and the yellows. This is what leads to the intensity of the blue hour. This absorption from the ozone layer filters out those middling colours, purifying the blue of the scattered light. It stops the colour of the high sky at this hour from being washed-out. It results in a deeper shade. One that calms the mind after a long, hectic day. One that reminds us that each dawn is a new page, and a new beginning.

Cumulonimbus with distant showers in the blue hour spotted Douglas Dubler over West Neck Beach, Long Island, US.