Thomas Hardy's Wessex was a peaceful spot. "Five decades hardly modified the cut of a gaiter or the embroidery of a smock-frock by the breadth of a hair. Ten generations failed to alter the turn of a single phrase." And as for the landscape, "the heaven being spread with this pallid screen and the earth with the darkest vegetation, their meeting-line at the horizon was very clearly marked".
Now if Hardy had looked more closely at his Wessex sky, he would probably have noticed that its pallor was largely confined to the zone immediately adjacent to this "darkest vegetation". We tend to think of the sky as being a uniform shade of blue, but inspected more closely subtle shades are evident; the blue gets lighter, paler - almost white, in fact - as the eye descends towards the horizon.
To understand why, we must recall why the sky is blue in the first place. Light coming from the sun is "white" light, a mixture of lights of all the colours of the spectrum. But light, as it travels through the atmosphere is subject to a process known as "scattering": it is obstructed by tiny particles in its way, and even by the molecules of the air itself, and deflected from its original path.
The scattering process is most effective in the case of "blue" light: the red and orange parts of the solar spectrum, and indeed all the other colours, for the most part surge straight through, virtually unaffected.
So when you look upwards at the sky, away from the sun, what you see is sunlight that was originally heading in an entirely different direction but has been scattered towards you by the atmosphere. And since the "blue" light is affected in this way to a much greater extent than any of the other colours, the sky, except for the sun itself, appears as blue. Near the horizon, however, matters are more complex. At this oblique angle, the scattered light approaches us almost horizontally through the air, and most of it must travel a much longer path than usual before it reaches us. Because of this long distance, the scattered blue light is subject to further attenuation on its journey; it is "re-scattered" before it reaches our eyes, and much of it is therefore extinguished.
But the small amounts of orange and red light which were originally scattered in the observer's direction are likely to survive this long journey with little further interference. In this way, the balanced proportions required to produce white light are virtually restored, and hence the pallor of the sky just above a distant horizon.