A vocabulary of landscape — a landscape of words
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shepherd’s warning
Shepherd’s warning is the second half of the old weather rhyme: A red sky in the morning is a warning, and a red sky at night is a delight. The physics behind it is real in the mid-latitudes, where weather moves west to east. A red sunrise means the sun is lighting the underside of cloud arriving from the west; a red sunset means clear air to the west with the cloud already gone past.
Etymology
The rhyme exists in English in shepherd and sailor versions and both are old, and a version of it appears in Matthew’s Gospel, where it is offered as something everyone already knows how to read. The color itself is Rayleigh scattering, doing what it does to light that has taken a long slanting path through the atmosphere.
Notes
The rhyme fails outside the belt of prevailing westerlies and in the tropics, and it fails when the weather is coming from an unusual direction, which is why it works about as often as it does and no more. The redness is intensified by dust and by aerosols, so the most spectacular examples follow volcanic eruptions and large fires and have nothing to do with tomorrow’s weather at all.