A Christmas Sunrise Surprise

Spectacular sunrises and sunsets often leave us elated: even more so in times as emotional as today. philip explains some of the science behind our memorable experiences.


It was still early and I was in the kitchen, making cups of tea and getting some of the food ready for our festive breakfast. Carols sang out from the radio and I did my best to ignore the news on this very different Christmas morning.
Our kitchen window looks northwards across a narrow valley on the edge of town and there is always much to see even if it is only a storm approaching from the west. That morning, though, I noticed something different, something special. The part of the eastern sky that I could see was suffused with orange light suggesting that we might be in for an interesting sunrise. This doesn’t happen very often here and I knew it wouldn’t last so I told Hazel, grabbed my camera and went into the street to get a better view. There was an unusual stillness, a rare quiet but, by contrast, the entire eastern sky appeared to be alight with a bright, fiery display that captured the view, transforming telegraph poles and nearby trees into skeletal silhouettes.
It was as though someone had taken a large brush and splashed paint in rough horizontal layers across the thin cloud that hung in the eastern sky that morning – starting with yellow, then switching to orange, then red and finally mauve.
By now Hazel had joined me and we stood there, neither of us dressed for the occasion but both in awe at the astonishing natural spectacle we were witnessing. I knew the colours were changing all the time as the sun crept upwards and the cloud cover shifted so I took a few photos as a record. Suddenly remembering where I was, I looked about and saw thick frost on the parked cars and realised I was getting cold. It was time to go in but I went with renewed optimism. Even in a pandemic year, perhaps especially in a pandemic year, the non-human world can surprise and thrill.
The rational part of me knows that there is a good scientific explanation for the extraordinary light show we witnessed but this does not detract from the spectacular nature of that morning’s sunrise. So, what is it about these displays that we find so captivating? The colours are surely part of this. The reds and oranges filling the sky express a certain danger, a wildness that is unpredictable, uncontrollable and ephemeral. Perhaps we also gain an insight into the power of the sun and a better appreciation of our place in the world as just one small part of the overall ecosystem?
As you might expect, the beauty and mystery of the light at sunrise (and sunset) have inspired artists who have tried to capture some of the effects in their paintings. Norham Castle, Sunrise, painted by the British artist JMW Turner in 1845 is a depiction of the morning light over this Northumberland landmark. The painting barely illustrates the castle itself, concentrating more on the light from the rising sun and its reflections across the nearby river. The French artist, Claude Monet was also fascinated by the effects of light at different times of day and created many artworks trying to capture these effects. One of his best-known depictions of the morning light is Impression, soleil levant 1872, showing the sunrise over the port at Le Havre with the sun casting red light across the water and orange light across the hazy clouds.
Science, on the other hand, provides us with a different understanding of the colours we see at sunrise. Two basic ideas are important here. Firstly, although the light leaving the sun appears white, it actually consists of light of different wavelengths that we see as a range of colours from red and orange through yellow and green to blue, indigo and violet. A helpful way to imagine this is to think of a rainbow where these different colours are spread out in the sky. Secondly, the sun’s light is scattered as it passes through the layer of gases, principally nitrogen and oxygen, that constitutes the atmosphere surrounding our planet. This scattering is wavelength-dependent so that blue wavelengths are scattered more than the red and orange.
With those two ideas in mind, let’s consider the sun in relation to the earth at different times of day. When the sun is high in the sky during the day, the sunlight will have a short path through the atmosphere. Preferential scattering of some of the blue light will occur making the sky appear blue and sunlight seem yellow. At sunrise, the position of the sun is very different. Sunrise occurs when the world turns until light from the sun just reaches the part of the planet where we are observing. With the sun low in the sky and close to the horizon, the sunlight will have to travel a much greater distance across the atmosphere. As a result, scattering away of blue light is almost complete, allowing the orange and red light to dominate. An analogous argument can be applied at sunset.
Although this explains how the light becomes orange and red at sunrise (and sunset), it doesn’t account for the variability of the event. This depends strongly on the particular weather conditions of the day. The key to a sunrise where orange and red light fills the sky, though, is high level cloud but not too much of it. This cloud catches the red and orange light, rather like a celestial projector screen, and the result is a memorable sunrise like the one I saw on Christmas morning.

Philip Strange is Emeritus Professor of Pharmacology at the University of Reading. He writes about science and about nature with a particular focus on how science fits in to society. His work may be read at http://philipstrange.wordpress.com/