As the afternoon stretches on the strength of the sun diminishes, the thermals fade away. There will be no more cumulus clouds today. The clouds weaken as their energy source disappears. At sunset the sky is clear once again.
The sun has dipped towards the horizon. It blazes from a shallow angle across the atmosphere, softening its rays into the golden hour, a time treasured by landscape photographers when the earth is bathed in rich yellow light and the texture of objects is revealed. The sun is now 5° above the horizon and different colours are starting to appear. The milky-white lip over the horizon of daytime is replaced by a yellowy red. Zones of red and orange and purple appear over it, almost in horizontal bands. As the sun sets in the west, in the east the earth’s shadow comes into view, a bluish-grey horizontal band, rising as high as 6° over the horizon.
On a clear day like this, the drama intensifies after sunset. Although the sun has gone, twilight fills the skies for several hours. Eventually the distinct bands of colour even into a purple hue of striking opalescence, ‘more pink and salmon coloured than true purple’1. This light remains for about an hour after sunset, until it is replaced by a cool blue glow 20° above the horizon. This is the twilight glow.
The further away from the equator, the longer twilight lingers. In the Orkney Islands north of Scotland twilight fills all the night hours from the middle of April to the middle of August and it is quite possible to read from a book or to garden at midnight. For twilight to properly end and night begin, the sun must be 19° beneath the horizon. Until this final tipping point light continues to be scattered, and it glows over the landscape. Soon, though, stars of different magnitudes are visible. High up in the mesosphere, fifty miles above ground, noctilucent clouds made up of ice crystals glimmer and twinkle like chandeliers. These are the highest of all clouds.
By now the heat of the day has faded away. The atmosphere is cold and clear. Down below in a meadow, blades of grass are cooling fast. On the stem of a dandelion, in a space so infinitesimally tiny that even James Glaisher or Francis Beaufort could never glimpse it, a tiny fleck of dew is forming. The start of another day.