The Big Question: Why did the Met Office get it so wrong?

Why are we asking this now?

In case it has escaped your attention, which is unlikely, this summer is rapidly turning into a washout. The bad weather has led the Met Office to revise its summer forecast saying that instead of the warm, dry weather it predicted in April, the beginning of August is likely to be unsettled and wet.

The misery of heavy showers, cooling winds and cloudy skies has dampened expectations that Britain could finally experience the sort of summer it had enjoyed a few years ago before the last two consecutive summers of wetter-than-normal weather. The Met Office's seasonal prediction in April did much to heighten expectations after the last two years of miserable summers.

What did the Met Office actually say in April?

It said that the coming summer is "odds on for a barbecue summer" and that temperatures are likely to be warmer than average and rainfall near or below average for the three months of summer. It predicted times when temperatures would soar above 30C, and although it did not rule out the chance of seeing heavy downpours "at times", it said that a repeat of the wet summers of 2007 and 2008 is unlikely.

Ewen McCallum, the chief meteorologist at the Met Office, put it more succinctly when he said: "After two disappointingly wet summers, the signs are much more promising this year. We can expect times when temperatures will be above 30C, something we hardly saw at all last year... we should be seeing some good hot spells and perhaps get the old barbecue out."

This seems quite definite and clearly wrong?

Yes, but the Met Office also put in plenty of caveats into its forecast, not least of which was the idea that the summer is only "odds on" to be barbecue-friendly. It wanted to get over the idea that any seasonal forecast, which this was, is always going to be couched in statistical probabilities. In fact, its scientists calculated that there was only a "65 per cent" chance of them being right on this, which means there was a 35 per cent chance of them getting it wrong.

Dr McCallum himself emphasised at the time that seasonal forecasts of this nature are still very experimental and there are always a lot of random events connected with the inherently chaotic nature of the weather that can blow a prediction off course. In this respect, the statements by the Met Office were not wrong, just perhaps unduly optimistic – although its April forecast may still turn out to be right.

But we are having a terrible summer – they got it wrong, no?

Not quite, or at least not yet. Although July was a pretty awful month, the weather in June was really rather pleasant with higher-than-normal temperatures, long sunny days and little rainfall. People tend not to remember further back than a week or two when it comes to the weather, and one thing that everyone should be able to recall is that this year's Wimbledon passed off with barely any rain – the new roof over the Centre Court was hardly used.

The other thing to remember is that the summer is not over yet. August could, even with an unsettled start, turn out to be dry, warm and sunny, a scenario that could fulfil the Met Office's April prophesy. As Mr McCallum said yesterday: "The jury's still out for most of August, which could still settle down. The patient's not quite dead yet."

Why did the Met Office make such a rash forecast?

A decade or more ago, the Met Office would not have made such a long-term "seasonal" forecast because the science of forecasting was simply not up to it. However, seasonal forecasts are going to be more common as weather forecasting in general gets better.

Forty years ago, weather forecasts were pretty unreliable, even over a one- or two-day period. In fact, a typical four-day forecast today is as accurate as a one-day forecast 40 years ago. However, short-term forecasts today tend to be very accurate, although the geographical position of the British Isles – a temperate, maritime climate of changeable winds – can still blow them off course.

The Met Office believes that the situation today with seasonal forecasts is about where we were 40 years ago with short-range forecasts. It is possible to give some indication of what is likely to happen over the next few months, but a more precise prediction is not yet possible.

How does anyone predict what the British weather is going to do?

Weather forecasting relies on three things: information coming in from different parts of the world about the weather, computing power to handle and process these data and the skill of human interpretation. All three have got better over time.

Data on changes in air and sea temperature, wind speed and direction, humidity, barometric pressure and cloud cover, combined with the explosive increase in computer power, have enabled scientists to build up accurate models of how the weather is likely to develop over a given area of land. And satellites have transformed the ability of meteorologists to gather data on the changing physical parameters that constitute the weather. They can cover huge areas of land and sea and can operate pretty much non-stop every day of the year.

What makes seasonal forecasting different from normal forecasting?

Seasonal forecasting involves analysing data over longer periods of time, and wider areas of the Earth's surface, than short-range forecasts. Scientists attempt to identify "signals" that could indicate whether a summer or winter period is likely to be wetter or warmer than usual. An important part of the process is looking at the changing state of the sea because it is now clear that one of the most important factors controlling long-term weather trends is the interaction between the ocean and the atmosphere.

But it will only ever be possible to give overall probabilities on the likely outcome of seasonal forecasts because of the inherent randomness of the world's weather system. When the "La Niña" ocean phenomenon of the South Pacific is active, for instance, a wetter-than-normal summer in Europe is four times more likely.

Is the Met Office ever going to be able to give accurate seasonal forecasts?

Yes...

* Satellites have transformed the ability of scientists to gather weather data

* Computers are increasing in power and creating better models of the weather

* We are only beginning to understand long-range weather forecasting

No...

* It made a mess of the previous two summer forecasts – in 2007 and 2008

* Short-range forecasts are very different from making guesses over many months

* The weather is too chaotic and random to ever make accurate long-term predictions