Weathermen don’t tell a lot of good jokes. The best I have heard recently came from Pete Buchanan, deputy programme manager at the Met Office in Aberdeen. It was a Friday, we were looking out of his window, and the sky was a flawless blue. “The forecast said rain,” I told him, firmly. I had checked. Also, my taxi driver had just cancelled a weekend caravan booking in response, and was fuming. It was cobalt out there, and azure, and turquoise, and a hundred other holiday-sunshine euphemisms. Of cloud, not a wisp. Buchanan skipped nervously to the window. “Ah,” he said. “Well, you could argue that blue sky means potential for cloud.” I laughed. An hour later, the skies were smeared and spitting. By the time I was heading back south on the train, it was gushing down.

Where did we ever get the idea that predicting the weather was boring? Your last raindrop probably felt pretty harmless. But before it got to you, its likely behaviour was being tracked, monitored and predicted by what is technically a special agency of the Ministry of Defence. He is a strange sort of beast, the weatherman; somewhere between a newsreader, a wizard and a spy. And except for those on television, he usually is a he. You will find him on hilltops and on boats, on army bases and in complex, flickering, com-puterised rooms. He might be in Afghanistan with the RAF, or one of 20 unenvied souls on the Falklands. He uses radar and satellites. In times of urgency, he has direct access to the Cabinet, sometimes even the Prime Minister. He gets up before 5am, and he plans your day. So why do we only notice him when he gets it wrong?

Wimbledon is the weatherman’s ultimate nightmare. When rain stopped play a few years ago, Des Lynam scrunched a piece of paper into a ball on air. “That’s what I think of the weather forecast,” he sneered, and threw it across the room. “Good theatre,” grimaces Tim Andrews at the Met Office HQ in Exeter, “but not very helpful.” This year, Andrews is heading the team responsible for predicting Wimbledon’s weather. “It’s very visible,” he says, “so we have to get it right.” Part of the problem is live television coverage. When weather stops play, the presenters still need to talk about something. “One thing we have achieved,” says Andrews, “is to have BBC weather presenters on the ground. I remember Philip Avery one year, live on air, telling Sue Barker what scattered showers were.”

During the run-up to Wimbledon, the Met Office will be providing three high-resolution forecasts a day, at 9.30am, 3.30pm and 8pm. That is Wimbledon time, rather than Met Office time, as every clock in the Met Office is set to GMT. Visit in British Summer Time, as I did, and you lunch a little late. By Wimbledon’s last week, they’ll be giving hourly updates and will be on call for the referees. In the short term, they rely on the rainfall radar. This shows the direction in which the rain has moved across Britain, and can be seen on the Met Office website, www.metoffice.gov.uk – if you want to see whether it is safe to have a picnic, take a look. Bear in mind, though, that a low-resolution online rainfall radar paints in broad swaths: rain over Northumbria, take an umbrella; no rain over Kent, don’t bother. The All England Lawn Tennis and Croquet Club is barely 1sq km. It’s no good saying, “It might be a bit wet this afternoon.” They need to know exactly when to throw down the covers. Even on the high-resolution rainfall radar in the Met Office, one pixel is 5sq km. “The life cycle of a shower is 20 minutes,” says Andrews. “You might have frequent showers; they might be moving fast. It’s a challenge.”

For a secure MoD base, the Met Office HQ in Exeter is a surprisingly open, friendly place. The building is bright and cheerful, and stretches down two sides of a long, gleaming courtyard. Staff call this The Street. They have eager little signs on the walls, which say such things as, “Did you know… we are developing a thermal model of the human finger to measure frostbite?” (No.) They have a gym and a book club. Every Thursday lunchtime, they have what they call the Street Briefing, in which one of the forecasters uses a projector to go over where they went right and wrong in the previous week. Virtually everybody wanders out to listen. Weather, most of them will admit, can take over a life, like owning dogs. When a weatherman’s wife takes in her washing, the neighbours tend to do the same. If the forecast is wrong, friends are tactful. And it’s not hard to go wrong: Britain, perched on the edge of continental Europe, has a changeable maritime climate, so is at the mercy of the weather systems that roll towards us, unhindered by the Atlantic.

Upstairs, you will find the operations room. It is almost silent, except for the tapping. “Business as usual,” shrugs Nick Grahame, who is the guidance unit manager and one of the chief forecasters (there are nine, who rotate). “When there is severe weather, the stakes go up. There would be more noise.” Here, they provide forecasts for the public, the government and commercial clients. At one desk, they are drawing maps for the BBC. At another, somebody is putting together the shipping forecast. Dogger, Fisher, German Bight; all that happens here. They are telling local councils when to stop gritting their roads, and the Environment Agency when to prepare for floods. They are watching mountains for climbers and seas for oil rigs. They are telling construction sites when it is safe to put up a crane and, yes, they are watching Wimbledon.

Sometimes, the government will get in touch with a specific request. After the freak London tornado of 2006, the Met Office was asked if such things could be predicted. “Not really,” was the reply. At best, on certain days, they could offer a 10-15 per cent chance that something similar might happen somewhere in the South East of England. “That’ll do,” came the response. The request turned out to be terrorism-related. If a house blew up in London, there needed to be a model for figuring out the likely reasons why. A statistician would call this “the suppression of uncertainty”. This is the sort of thing that makes passing lifestyle journalists feel somewhat frivolous. They also get a lot of telephone calls from concerned members of the public, particularly during floods, winds, or heat waves. When that mini-earthquake hit the UK in February, the telephones went wild. Staff were a little nonplussed. Earthquakes are not weather.

If they have a power cut at the Met Office (and when I was there, they did), services are transferred to the back-up HQ in Aberdeen. The exception to this happens on the two strips of desks that make up one of the two World Area Forecast Centres, which exist to send forecasts for aviation purposes around the world. The other is at America’s National Oceanic and Atmospheric Administration in Kansas. According to one pilot from a major airline, these forecasts tend to be “uncannily accurate”. This is not so surprising. The vast majority of aircraft have meteorological instruments strapped on to the outside, all transmitting data home. Through these, from a monitor in Exeter, you can see a little dot representing almost every aircraft in the world.

“I didn’t know that,” says my pilot, surprised, “but it makes sense. We used to have to take readings, and send them over the radio.”

Older weathermen used to go home every night with their hands covered in ink. Observations were drawn on to maps by hand in black and red; forecasters would add the isobars. Sometimes these would be fed into a fax. Sometimes, somebody would jump into a Jeep and drive to the nearest radio station. These days, downstairs, they have the supercomputer hall. The equipment there can do in about half an hour what 8,000 PCs might do in a month.

Grahame, my guide, was always into weather. He used to make meteorological records as a teenager, studied physics and meteoro-logy at Reading, and joined the Met Office in 1979. He started forecasting in the late Eighties. All forecasters are meteorologists, but not all meteorologists are forecasters. Before that, Grahame worked in other areas: programming, the upper atmosphere, climate cycles of 9,000 years ago… At the Met Office, the head swims easily. Most of the younger generation have backgrounds in maths or the more mathematical sciences. Contrary to popular supposition, a geography degree isn’t that much help.

They value accuracy at the Met Office. This means you rarely get a straight answer, or rather, what you or I would class as a straight answer. Ask a Met Office forecaster if it will rain, and he will tell you about humidity, wind speed, pressure and temperature. Press him, and he might tell you that, of the 24 strands in the Mogreps global ensemble modelling system, a majority appear to coincide in their indication of precipitation, while a minority diverge. Threaten him and he may concede that there is an 82 per cent chance of rain, say, and that he is 73 per cent confident in that prediction.

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