We can't quite remember when the forecasters got it so wrong however, what should have been a day of high temperatures and bright sunshine is instead more like an Autumnal day in London.

What started off as drizzle has intensified into a downpour while the temperature is unseasonably cool.

Consequently, everything we have learned in the previous four sessions goes out the window, with Pirelli boss Paul Hembery admitting that this afternoon will be a massive challenge for all involved.

"It's a big challenge for the teams, more so because this track is quite difficult with setup," he told Pitpass. "Over the last couple of days the teams have been doing a lot of work on trying to find the right balance for the car, they've been suffering with the movement from understeer to oversteer. Maybe too much oversteer in some cases. To throw in maybe a damp equation…"

The GP2 race started in damp conditions however, within 15 laps a dry line was becoming evident. "If it started like the GP2 race they'd be on the intermediates," Hembery continued. "Looking at the way it's drying they would then be jumping on to the slicks rather than using the full rain tyres."

Crossover from the intermediate tyres to slicks is expected to be around 115% of a 'normal' lap time, though some teams could gamble and switch earlier. "Supersofts will work pretty well in lightly damp conditions," said Hembery explained.

However, when teams make the decision to switch from intermediates to wets, if indeed it is still damp come race time, will be a step into the unknown. There's been no wet running throughout the weekend, and while the teams now have some experience in the rain with Pirelli rubber, they've not yet experienced it with the supersoft compound.

"We haven't got the experience of that crossover with the supersofts, we've not had the rain and the supersofts before, we've just had rain with soft tyres," said Hembery. "That will be, I'm not going to say a jump in the dark for teams, but that will be an area where they haven't got experience and they'll have to base it on good judgement.

"The track seems to be draining pretty quickly, and if we do get a bit more rain later on in the day like we've had this morning I'm confident we'll have a similar situation where they'll start on intermediates, the track will dry quickly, and they'll jump straight on to the supersofts."

Should the conditions remain as is, it's bad news for Ferrari, but good news for McLaren, with Jenson Button eyeing the possibility of celebrating his 200th Grand Prix in the best way possible… with a win.

In the hour leading up to the start the weather is constantly changing, it's raining, it stops, it's drizzling, it stops. It's certainly very cloudy with little chance of sunshine. It's particularly wet in S2 where the concrete run offs have a worrying sheen. Turn 1 is more greasy than damp.

The one driver who could make a brave gamble today is Sebastien Buemi who starts from twenty-third. The Swiss could opt to start on slicks and, should conditions improve, gain a significant advantage on his rivals.

Five minutes before the pitlane opens, with it still drizzling, Mike Gascoyne confirms that both Lotus' will go to the grid on inters.

As a number of teams follow the Lotus' example, drivers complete a couple of installation laps - courtesy of a trip through the pitlane - in order to evaluate the track conditions.

Renault reports that it's "quite damp out there", Ferrari claims that the race will start under "light rain". On track, as if to prove the point, Massa is sliding all over the place at Turn 4. It's also very windy.

Whatever the conditions at the start, pole-man Vettel will be keen to get ahead of the pack and leave his rivals to sort themselves out, especially in that greasy first turn. Then again, Hamilton, Button, Massa, Alonso and Webber will have similar thoughts.

Speaking minutes before the pitlane closes, Paul di Resta insists that it's not wet enough for wet tyres, that they would destroy themselves, and that inters are the way.

Worryingly, moments before the start of the race the live timing appears to be malfunctioning.

The field heads off on the warm-up lap, everyone seemingly on inters. Some parts of the track clearly wetter than others.