Our ancestors predicted the weather by looking at the sky, or by watching the behaviour of animals or plants. Weather lore, indeed, in the form of popular sayings that have been handed down through generations, is to be found in almost every culture in the world.
Careful analysis suggests that this weather lore can be conveniently divided into seven separate categories. First come sayings based on the current behaviour of the elements or the appearance of the sea and sky. "Red sky at night, the shepherd's delight" is a typical example. In the second category are those sayings concerned with plants and animals - the assumption being that certain living things can intuitively predict the weather by some means that we cannot understand. And a third type of weather saying deals with the aches and pains of those afflicted with arthritis, rheumatism, or less serious ailments. Jonathan Swift provides a good example here: "A coming shower your shooting corns presage".
The fourth and fifth categories are almost contradictory. The former might be called "persistence" - the notion that a series of rainy days must be a sign of yet more rain to come, or that good summers always come in twos. The fifth, on the other hand, brings into play the law of averages: adherents would argue, for example, that if we suffer a harsh winter, we will be rewarded by a fine warm summer later in the year, or that a good summer one year must be paid for by a dreary season 12 months later.
The sixth class contains the cycles, where people have claimed to have discovered repeating patterns in our weather related, for example, to sunspots, the phases of the moon, or some period of years. And finally, in category seven, we have the multitude of proverbs related to saints' days or other notable features of the calendar: of these, the legend of St Swithin, is perhaps, most widely known.
In general, any saying that purports to give a long-range forecast - as in the case of St Swithin's 40 days - should not be taken seriously. Sayings, on the other hand, that predict the short-term prospects - particularly those based on the appearance of the sky - need not be discarded quite so lightly. Some, indeed, have a subtle logic to them: "rain before seven, dry by 11" reflects the fact that most bands of frontal rain have a duration of only a few hours. And "a red sky at night" suggests that the atmosphere over the horizon to the west is free of cloud - usually a fair indication that rain is unlikely for 12 hours or so.