FROM FOLKLORE TO FORECASTS 2
As physicist Niels Bohr said (and it wasn't original then), 'Prediction is very difficult, especially about the future.' The difficulty of foreseeing what is to come is not always a problem for science. We can, for example, predict exactly when Halley's Comet will return and sweep around the Sun on its 75-year orbit (the next time it will be sighted from the ground will be in 2061), but we are incapable of forecasting the weather more than a few days ahead.
Knowing how the weather is going to develop is incredibly important, but how can we know the future when dealing with something as intangible as this? In principle it seems it should be feasible. Ever since Newton gave us a mechanical picture of the universe, it has seemed possible that predictions of the future could become near-perfect. The eighteenth-century French academic Pierre-Simon Marquis de Laplace noted that because of the apparently clockwork-like nature of everything, if a hyper-intelligent being could have access to all the data in existence 'nothing would be uncertain and future, as the past, would be present to its eyes'.
First attempts
Until recent times, even the vaguest possibility of Laplace's vision has been inconceivable for anything other than a deity. Yet forecasting how the weather will change is a challenge that has been faced for as long as human beings have thought about nature and how it works. Before a scientific approach was possible, predicting the weather was largely left to the priest or the shaman. At first sight, what they did appears to be not so much a prediction as an attempt at weather control. The idea was to intercede with the gods who were thought to inflict the weather on the people of the Earth as a reward or punishment for their actions.
In practice, though, a savvy priest could indeed be capable of spotting signs that helped predict the weather. If he or she had a fair idea of what was coming, they could then claim to be asking the gods to bring bad weather as a punishment or good weather as a reward. If they got the prediction right, they would appear to wield enormous power, keeping the priesthood safe in their position of esoteric knowledge.
This supernatural view of the weather as something caused by gods, spirits or magic would last well into the medieval period, when witches were sometimes blamed for bringing on a patch of bad weather that damaged an opponent's crops (something that sadly can still happen in parts of the world). As with many topics, one of the first to think scientifically about the weather was the Greek philosopher Aristotle, who lived between 384 and 322 BCE. We now know that Aristotle was often wrong about science - but in a sense that doesn't matter.
The problem with the proto-science of Aristotle's time is that it worked more like the law does today than a true science. Instead of relying on experiment and careful observation to decide the most likely scientific explanation for a phenomenon like the weather, different theories would be debated, and whichever idea came out best in the debate would be accepted as the truth, even if it bore no resemblance to reality. Infamously, Aristotle opined that women had fewer teeth than men. He never bothered to check and people took his word for it.
Aristotle's scientific legacy was not so much in the accuracy of his theories, as his ability to set the agenda. He covered a vast range of subjects that science would come back to again and again. Some of the topics were abstract, such as the nature of infinity, but others were very practical, with significance for the everyday lives of everyone. And one such topic was forecasting the weather.
Aristotle discussed one of the fundamental components of the weather system: the flow of water through the atmosphere and around the world. He described how water evaporates from bodies of water like oceans and lakes, is carried through the air by the wind and then falls elsewhere as rain. His concept might have been vague, but it did present a basic picture of a key aspect of weather. Aristotle made this the subject of a book called Meteorologica, the word from which we get the clumsy name 'meteorology' for the study and forecasting of the weather. It sounds like it should be about meteors, but it originally just meant the study of things that are raised up and lofty - in essence what goes on in the air (though in Aristotle's original version it did indeed include those meteors).
The ancient Greeks were largely interested in describing nature, exploring its qualities rather than applying any kind of quantitative measurements. So, for instance, like many early cultures, they used wind vanes to see how the wind varied in direction, but they didn't think about making a numerical measurement of wind speed. Probably the best-known surviving piece of Greek weather technology is the Tower of the Winds, a 12-metre (40-foot) high structure in Athens, devised by the astronomer Andonikos around 50 BCE and originally topped with an ornate weathervane.
Apart from minor local developments, it wasn't until the seventeenth century that Renaissance thinkers began to look for ways to make direct measurements, pinning down the specifics of weather. How much rain had fallen? How fast was the wind blowing? How did temperature vary throughout the day? Was there any relationship between the air pressure and what was happening to the weather? Were there any regular patterns, making it possible to predict how the weather would change in the future from its current state?
As basic science began to explain the relationship between temperature, pressure and volume of a gas like the air - and at the same time instrument makers added thermometers and barometers to the armoury of would-be weather forecasters - it became possible for the first time to get a feeling for what the weather was actually doing at the moment, and from this to make a stab at predicting how it would change in the future.
The Tower of the Winds.
George E. Koronaios, CC BY-SA 2.0
Reading the sky
Attempts had, of course, been made before. Folk weather forecasting made use of (and still does) cloud patterns, plants, seaweed and the behaviour of animals and birds to predict what was likely to come. Much of this lacked a scientific basis, but occasionally they got it right. Many people are familiar with the rhyme 'Red sky at night, shepherd's delight; red sky at morning, shepherd's warning', suggesting that good weather followed a red sky around dusk (producing happy shepherds), but a red sky at dawn was likely to presage a storm.
Variants of this weather saying have been around for a long time. For example, in the Gospel of Matthew in the Bible, dating back to the last part of the first century CE, Jesus is asked to show a sign from heaven. He replies, 'When evening comes, you say, "It will be fair weather, for the sky is red," and in the morning, "Today it will be stormy, for the sky is red and overcast." You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times.'
The durability of the red sky rule of thumb reflects an element of scientific basis. Red skies appear for the same reason that the setting sun appears red, while the sky unencumbered with clouds generally appears blue - because of the scattering of sunlight. The Sun's light is white - a mix of the colours of the rainbow. On the whole, the molecules of the air (and dust particles) scatter more of the blue light in the mix than red, bouncing light away from its usual straight-line progress. The result is a blue sky because the blue light from the Sun is less likely to come directly to your eye and more likely to take a roundabout trip across the heavens.
With the bluer aspects of its white light extracted and sent off on an excursion through the atmosphere, the Sun appears yellow in the day and red in the evening, when the light has to go through more air because of the shallower angle of its rays, thus more light is scattered. Red skies most frequently accompany high atmospheric pressure, which has a tendency to trap more scattering particles in the air. High pressure tends to turn up at night if it is moving into a region in the northern hemisphere, where winds from the west prevail, while high pressure in the morning is more often on its way out. As high pressure over an area tends to make for better weather, this results in a degree of success for this folk prediction.
There are also certain types of cloud that have been used to forecast what's on the way, with some good cause. Of course, sighting a huge black thunderhead moving in isn't exactly rocket science, nor is the link between darkness in the clouds and rain, but there are more subtle indicators. Altocumulus clouds in a pattern rather like lines of fish scales (sometimes called a mackerel sky) and comma-shaped high cirrus clouds, known as mares' tails, both indicate a good chance of high winds on the way, as these clouds often form in advance of a storm.
Nature and legend
Relying on nature for short-lived local forecasts also has some merit. It is still said today that if cows are lying down during the day, it is likely to rain, because they prefer not to lie on wet grass so get settled before the rainfall sets in. They do often seem to read the signs well, though there are plenty of other reasons a herd of cattle could decide to settle down other than an impending downpour. Similarly, swallows are said to presage stormy weather...
