Why is the sky blue?
Who cares? is one answer.
It’s not always, is another. Sometimes it’s grey, or red, or black.
Red sky at night, shepherd’s delight. Blue sky at night, it’s morning.
In the Antarctic the sky is perpetual blue – day and night – from October till February, because the sun never sets in the summer months.
If the question is a serious one, the answer is, Because of sunlight scattering in the atmosphere:
- Sunlight (white light, made up of seven primary colours) hits the atmosphere and is scattered by atmospheric particles (nitrogen and oxygen molecules).
- Blue light is scattered the most, due to its shorter wavelength, ‘colouring’ the sky we see. The other colours pretty much come straight on through, still essentially combined as white, illuminating what we see all around us.
- At sunset (and rise) the obliquely incident white light has much further to travel through the atmosphere.
- Now the blue wavelength is scattered over and over, and loses its intensity.
- The reds and oranges, which scatter less, become the dominant hues, and the sky seems red/orange. The effect is even more marked when there are larger scattering particles in the air like smoke or dust .
In 1815 the Indonesian mountain Tampora blew up. A hundred thousand people were killed by the blast and associated tsunamis. A similar thing happened in 1883 when Krakatoa blew (again, in longsuffering Indonesia).
In both events the smoke, dust and ash from a volcano on the opposite side of the world diffused into and through European and American skies – travelled right around the world, in fact, shielding much of the sunlight, cooling the planet, and portending what were termed ‘years without summer’ in 1816 and 1884.
In 1816 (Bill Bryson writes in his A Short History of Nearly Everything), “Crops everywhere failed to grow. In Ireland a famine and associated typhoid epidemic killed sixty-five thousand people … In New England … morning frosts continued until June and almost no seed would grow … livestock died … .” And so on (see https://en.wikipedia.org/wiki/Year_Without_a_Summer).
Similar consequences after Krakatoa.
The first Europe heard of the Tampora event was about seven months after, news being slow to travel in those times, but even by then, artists were using different pallets to colour their skies. Browns and oranges more prevalent. Without knowing why, artists recorded the changes before newspapers did.
News travelled more quickly in 1883 (and, indeed, the Krakatoa eruption was heard up to three thousand miles away, and ‘barographs’ recorded the shock waves rippling around the earth four times), but European artists again painted their skies differently. There were no colour cameras then, so no ‘hard’ record of the events, but even today we are able to ‘see’ them in paintings of the time. Immerse yourself in the following paintings by William Ascroft, 1883, and feel the bloated, oppressive English sky.
Nature, in all her awesome, sinister glory.
So, why is the sky blue? Well, sometimes it’s really not.