At midday the cumulus cloud is drifting on a current through a pale blue British sky. This shade of blue is unmistakably British. At southern latitudes, in Italy or Spain, the sky has a different hue. In Lima or Cairo or Sydney, it is different again.
The sky has no physical reality. Its blue tint is nothing more than a scattered light show that reaches the earth eight minutes and twenty seconds after being emitted by the sun. When sunlight arrives in our atmosphere it exists as a combination of colours which make up white light. It then travels onwards through the five principal layers: the exosphere, the thermosphere, the mesosphere, the stratosphere and, finally the troposphere, where almost all organic life exists and weather occurs.
The blue we see is the scattering of sunlight as it strikes against air molecules or other particles as it passes through the atmosphere. Each collision splits the white light into its constituent colours – red, orange, yellow, green, blue, indigo and violet – but it does not do so evenly. Every colour has a distinct wavelength – the highest, red, at 710 nanometres, the lowest is violet at 400 nanometres. The molecules in our atmosphere are more effective at scattering low-wavelength light like violet, indigo or blue. As the human eye has not evolved strong sensitivity to violet, which is the dominant tint in the sky, we see a vaulted blue space above us. The process can be imagined as billions upon billions of distinct explosions in the atmosphere, each a firework of bluish light.
The different blues of skies across the world are the product of the atmospheric composition in that particular place. From sea level the sky is bluer near the equator. The bluest skies on earth are said to be over Rio de Janeiro in Brazil. At northerly latitudes when the atmosphere is thinner the skies have a whiter tint. Altitude has an effect too. Two thousand feet up on the plains of Castile, Madrid has distinctive hard blue skies. Even darker hues, lapis or Prussian blue, are seen by Himalayan climbers or balloonists. In every case the deepest blue is seen at an angle of 90° from the eye to the sun.
Other combinations of colour can be generated by pollution. Smoke particles are much larger in size than air molecules and they intensify the scattering of light at all wavelengths. Polluted cities can have off-white skies with barely a trace of blue in them. This same colour can be seen if you look dead ahead. You are now looking through a much greater thickness of atmosphere, and as a result the blue has disappeared completely to be replaced by the familiar white haze of the horizon.
The pale blue British sky is a product of our humid, or damp, atmosphere. The greater concentration of water vapour particles increases the scattering of sunlight, weakening the tint. Sometimes a rain shower changes everything, clearing the atmosphere of dust and vapour and reinvigorating the skies: making them a lucent, fresher blue.