After the wettest June on record, better weather is promised - but not just yet

On Tuesday hailstones the size of 20p pieces smacked into the streets of south London; later in the week, officials at Wimbledon considered extending play into a third week because of the miserable weather; meanwhile, much of northern England is recovering from a deadly spate of heavy flooding.

The apocalyptic June weather has even led some people, notably former weather presenter John Kettley, to suggest that the British summer was over in April.

The Met Office confirmed this week that it has been the wettest June since records began in 1914. But reports of the death of summer have been greatly exaggerated: despite inherent uncertainties in making long-term weather forecasts, the Met Office says that Britain will enjoy a relatively warm, dry summer in the coming months, although more unsettled weather lies ahead for the next few weeks.

"The heavy rainfall we have seen through June has been due to the jet stream across the Atlantic - a ribbon of strong winds in the upper part of the atmosphere - being much further south than it would normally be,"
said Dave Britton of the Met Office. "This jet stream steers weather systems across the Atlantic. As it is further south these are being pushed across the UK rather than steered further north between the UK and Iceland." Britain's rainfall average last month was 134.5mm (5.3in), beating the previous high in 1980 of 121.2mm (4.8in).

The country's changeable weather is a result of its location - with the Atlantic on one side and a large continent on the other, subtle changes in the wind direction can bring marked changes in the weather, making predictions difficult. According to the Met Office, the "UK also lies near the 'battleground' of warm air from the tropics and cold air from the poles, which spawns the vigorous depressions and quiet anti-cyclones that also bring marked changes in weather."

Weather forecasting begins with a detailed measure of the current state of the atmosphere. Scores of instruments on weather balloons, satellites and ground observatories around the country make continuous measurements of sunshine, air pressure, wind direction and temperature. The numbers are then fed into a mathematical model running on a supercomputer that is roughly equivalent in power to 8,000 desktop PCs.

"They do lots and lots of simulations with slightly different initial conditions just to see the range of possible outcomes, called an ensemble," said Ralf Toumi, an atmospheric scientist at Imperial College London. "From the ensemble they deduce the most likely outcome."

It is not an exact science because weather systems are governed by chaos theory, which says that small changes can lead to highly unpredictable disruptions later down the line.

The Met Office carries out forecasts for the next five days, which are available to the public. The European Centre for Medium-Range Forecasts, a collection of all the meteorological centres across Europe, also carries out medium-range forecasts for five to 10 days. "The errors grow exponentially - you will not find credible forecasts even for the large scale that go past 10 days," said Dr Toumi.

The Met Office also advises on likely trends up to 15 days ahead and provides an outlook for the month ahead. "We have recently begun producing seasonal forecasts. Like the monthly forecasts these do not say whether a specific day will be good or bad, wet or dry, but give an idea how the season may be in terms of temperature and rainfall against long-term averages," said Mr Britton. In the past decade computers have become more powerful. The Met Office says its forecasts for the day ahead are correct six times out of seven, and its three-day forecasts are as accurate as one-day forecasts were 20 years ago.

As for reports of the demise of summer, Mr Britton urges some optimism. "I wouldn't write the summer off yet -there's a reasonable chance of seeing some spells of good weather into the summer. We've got two months to go yet."