Ahead of today's first free practice session, the air temperature is 18 degrees C, while the track temperature is 17 degrees. Following lots of rain overnight, and a further storm just a couple of hours ago, the track is still damp with the dark clouds overhead suggesting that there will be more to follow. Indeed, Timo Glock is advised that a big band of rain is heading his way.

Tyre compounds this weekend are hard and soft however, it is unlikely that we'll be seeing much of the slicks today.

Despite the fact that the teams had to shut down for two weeks of the three-week break, there are a significant number of upgrades here this weekend.

The car that has been most heavily revised is the Renault, the French team running its version of the F-duct for the first time. Ferrari also has a number of upgrades including a new floor and a new diffuser while Mercedes has revised its F-duct.

While McLaren has a fearsome reputation for its development programme the Woking outfit has just a few new parts on the MP4-25 this weekend while its rivals from Milton Keynes have a new rear wing in addition to the controversial front wing used with such awesome effect in the previous couple of races.

The FIA has revised the test for flexibility in the front wings - with a nod to Red Bull and Ferrari - but both teams have passed, much to the frustration of their rivals.

When the lights go green Timo Glock is first out, with Alguersuari, Yamamoto, Buemi, di Grassi among the early risers.

Ten minutes into the session, with all but the McLarens having gone out for an installation lap, it starts raining again. Despite that, Glock comes up for a second run, wisely lifting off through Eau Rouge.

The Virgin driver posts the first time of the day, crossing the line at 2:06.752. Elsewhere, Hamilton heads down the pitlane.

Realising that the 'wet stuff' could be here for the weekend, more and more drivers head back to the track including the Red Bull duo. Going into La Source, the rear of Vettel's car snaps out of shape under braking.

Di Grassi posts 2:12.357 to go second but is leapfrogged by Vettel who crosses the line at 2:09.693.

A 2:05.845 sees de la Rosa go quickest, like his rivals the Spaniard is on the extreme wets.

Thirty minutes into the session, Sutil goes quickest (2:03.331), ahead of Hamilton, Hulkenberg, de la Rosa, Alguersuari, Kobayashi, Buemi, Glock, Vettel and di Grassi.

The conditions are constantly changing - it rains, it stops, it rains again - but essentially it remains very wet. Nonetheless, Alonso raises the benchmark with a 2:02.582.

Vettel goes quickest with a 2:02.450 as teammate Webber improves to fifth with a 2:02.926.

At 'half-time', it's: Alonso, Vettel, Sutil, Button, Webber, Hamilton, Massa, Hulkenberg, de la Rosa and Alguersuari. All but Barrichello and the Renault drivers have posted times.

As he heads into the pitlane, Alonso confirms that it is "raining quite heavily at the other side of the circuit". He heads through the pits and performs a practice start at the top of the hill.

As Schumacher seeks to improve on eighth (2:03.489), a bolt of lining rakes the sky above La Source. It is also getting increasingly dark. Indeed, Bob Constanduros texts to say that the light is similar to Singapore.

As the rain intensifies the drivers realise that there is little to be gained from continuing, as they all return to their garages an eerie silence descends on the Ardennes.

With twenty-five minutes remaining, Schumacher breaks the deadlock, followed by Alguersuari.

"Blue bits to the south over Stavelot," texts Bob Constanduros, referring to the newly named Curve Paul Frere . In the Red Bull garage Vettel chats with Adrian Newey.

With fifteen minutes remaining, the Mercedes crew are changing the floor on Rosberg's car. Sakon Yamamoto is the only driver on track.