Why is it so hard to predict the weather a week in advance, and how can scientists tell us what they think the climate will be like in 50 years’ time?
First of all, it’s important to understand that weather isn’t random, it’s chaotic. If the weather was random, it would mean there’s no possible way of knowing what it was going to do next. However, the weather does obey the laws of physics and every change in the weather has a cause. The problem is that since there are so many possible causes, we can’t know about them all.
You may have heard of the butterfly effect (first proposed by Ed Lorenz in the 1960s): A butterfly flapping its wings in the Amazon rainforest might, through a long line of unlikely but possible consequences, cause a storm over Texas. In a similar vein, if we don’t know what’s going on in the atmosphere and on the Earth’s surface down to the detail of a butterfly flapping its wings now, we can’t hope to know how that’ll affect the weather in a week’s time. The possible range of consequences grows with time – and the ability to accurately forecast the weather will decrease with time. Of course, some weather situations – such as High pressure, are much easier to predict than others – such as the snow showers which can be caused by Arctic maritime air, but in general tomorrow’s weather forecast is much more likely to be accurate than one for 10 days’ time.
Animation created by Ross Bannister
This animated double pendulum illustrates the chaotic nature of weather really well: The pendulum starts off in the same position, but with a slightly different rotation speed (400.0 degrees/ second v. 400.1 degrees/ second). Over time the difference in the way the double pendulum rotates grows, until the two are behaving completely differently.
Modern forecasting techniques try to capture the range of possible future weather by making an ‘ensemble’ of weather forecasts – rather than making one forecast with one set of starting conditions (the weather now) they make many forecasts, each with tiny differences in the weather now – trying to take into account the effects of all the possible ‘butterflies’ or other tiny details about the climate system that we can’t possibly measure. The ensemble of forecasts gives forecasters a range of possible weather forecasts, with some indication of what’s most likely, and what might happen.
The climate, unlike the weather, is not chaotic. Remembering that climate is ‘average weather’, if large scale factors which control the climate are known – the composition of the atmosphere, the location of the continents, the Earth’s position in relation to the Sun etc. then it’s possible to predict the climate.
In between weather forecasts and climate forecasts come seasonal forecasts – the ‘what will the weather be next winter’ type questions. As the weather is chaotic, this is very hard to do, but there is some skill to be found in looking at the large scale influences on the weather – for example, if there’s a strong El Niño occurring, then certain weather patterns are more likely to form than others.
The North Atlantic Oscillation (or NAO) is another of the many factors which can be looked at. Meteorologists look at the pressure difference between Iceland and the Azores. The pressure is always lower in Iceland than in the Azores because of the large scale circulation of the atmosphere, however the difference in pressure can vary. A large difference in the pressure (a positive NAO) leads to stronger westerlies, bringing moist air to Europe. Consequently, summers are cool and winters are mild and wet in Central and Western Europe. In contrast, if the pressure difference is small (a negative NAO), westerlies are suppressed, winters are cold and dry in northern European areas and the depressions track southwards toward the Mediterranean Sea, bringing increased storm activity and rainfall to southern Europe and North Africa.
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