I remember many years ago down in Exeter someone said to me " I never watch the weather forecasts because they are always wrong" I found difficulty in making him understand that if he never watched how did he know we were always wrong. I had even less chance of persuading him that if we were always wrong, by going the complete opposite to the forecast, he could always be right.
All through recorded history we have tried to forecast the weather. Initially using the weather proverbs right down to the present day with super computers, but modeling the atmosphere and the interconnected oceans is so complicated that we still have a long way to go.
Traditionally the meteorologist gathers in weather data twenty-four hours a day from all around the world, including observations from land sea and in the air. This information is fed back very quickly and put into the super computers, who, by solving a series of mathematical equations give the forecaster different weather maps up to a week or so ahead. They provide only one solution to the evolution of the weather patterns. This is called a deterministic forecast and gives little idea of the probability of certain events occurring. So scientists tried to find a way of giving the forecaster the probability of the deterministic forecast being correct; this they called an ensemble forecast.
In the atmosphere the Chaos theory reins supreme. This states that if a butterfly flaps its wings in Japan it can have an effect on our weather in the UK. It is probably easier to understand, from the weather point of view, with a different example. If a raindrop falls onto the Andes in South America, where the mountains can have a very sharp pointed top, and there is a light easterly breeze, it will blow the rain onto the western side of the mountain where it will eventually flow into the Pacific and interact with the weather systems there. If, however, there is a gentle westerly wind the raindrop will then fall onto the eastern side and become, eventually, part of the Atlantic weather systems.
To cater for this very small change in the initial position of this raindrop, ensemble forecasts run the deterministic forecast many time using slightly different starting points ending up with many different forecasts for a few days ahead. This may seem a complete waste of time, but it isn't, because it then allows the forecaster to determine the probability of certain weather events happening.
If in the end results most of the forecasts are close together, then the meteorologist can predict that the atmosphere is behaving itself and has a high degree of confidence in the forecast. If, on the other hand, the ensemble forecast has a wide scattering of results, then the confidence in that particular forecast is much lower.
In this way the forecaster can give the listener or viewer a probability of the forecast being correct. And I shall be writing more about probability forecasts later in this series.
Related Links:
- Find out more about weather around the world
