Ahead of today's sole free practice session, the air temperature is 19 degrees C, while the track temperature is 20 degrees. It is bright but there is a lot of heavy cloud hanging over the hills that surround the Principality.

There was rain overnight and earlier this morning, consequently, the session has been declared "wet", with official estimating an 80 percent chance of rain in the opening fifteen minutes of the session. Most drivers come out on intermediates.

Fisichella is first out, followed by Nakajima and just about everyone else. Only the Ferraris, Toyotas and Barrichello stay put.

Five minutes into the session, as the rest of the field heads back to the pits, Massa comes out. The Brazilian, having completed an installation lap, continues around to post the first time of the day, 1:23.729. Moments later, Raikkonen heads down the pitlane.

Massa raises the benchmark to 21.359, as Trulli and Glock finally come out to 'test the water'.

As Nakajima becomes the second driver to post a time (26.714), officials announce that there is an 80 percent chance of rain in the next seven minutes.

Rosberg goes second (21.683) ahead of Raikkonen (24.990), as Massa nearly loses it as he passes Kovalainen at the Nouvelle Chicane. Nonetheless, the Brazilian gets down to 19.836.

There's a clear dry line appearing, but how ling before it's wet again?

A 19.204 sees Raikkonen go quickest, as Glock goes quickest in the first sector. Massa and Barrichello also appear to be on hot laps.

Sure enough, Massa re-takes the top spot with a 18.418, as Rosberg goes second (18.590). Webber goes fourth, ahead of Hamilton, Glock and Barrichello… it's very, very busy out there.

No sooner has Kovalainen posted a 18.325, than Massa improves with a 17.959, maintaining his place at the top of the timesheets.

Fisichella pulls off track at Mirabeau, as Raikkonen improves again, the Finn crossing the line at 17.585.

Such is the speed with which things are changing, Hamilton has already slipped down to seventeenth, moments ago he was fifth.

Lo and behold, the Englishman posts a 17.084 to go quickest, as his teammate takes second with a 17.468.

Despite the damp track, and the threat of further rain, the Englishman seems supremely confident. Meanwhile, Kubica goes fourth (17.687), ahead of Massa, Button and Nakajima.

Twenty minutes into the session, Coulthard is the only driver yet to post a time.

A 17.177 sees Raikkonen split the McLarens, as Sutil improves to seventh. The German's Force India teammate, Giancarlo Fisichella, appears to have got going again. That said, the Italian slows dramatically at Portiere and almost collects Bourdais.

Kovalainen takes the top spot with a 16.567, as Rosberg improves to fourth.

Kovalainen loses the back end of his McLaren at the Swimming Pool exit, his rear end clouting the wall with enough force to damage his right rear wheel, and probably the suspension. As the Finn sheepishly explains what happened over his radio, his crew politely advise him that - thanks to the fact the back of the pits overlook this particular stretch of track - they have seen everything. "We know Heikki, we know!"

After a relatively long delay, the session is red flagged while the silver car is removed. Heikki walks back to the pits, his helmet still on his hear… ergo, it was his mistake.

At this point - just after half time - umbrella are raised in the stands, the rain begins to fall. Only fifteen minutes late, but why can't we have these people producing our national weather forecasts.

"It was a little mistake, I went too fast," says Heikki. "Asked if the rain had played as part in the incident, the Finn is refreshingly honest, "not at all," he says, "it was my mistake".

The session resumes after just a couple of minutes, but with the rain falling there's no rush to get back to work.