Ahead of today's opening practice session the air temperature is 17 degrees C, while the track is 18 degrees. It is unusually overcast and cool, with widespread predictions of rain at some stage during the course of the session.

Tyre options this weekend are medium and soft, though Pirelli is experimenting with a new hard compound today.

While the championship battle remains the focus of attention, there is also intense pressure on McLaren, following a couple of very disappointing outings, and Lewis Hamilton, who celebrates his one hundredth GP outing this weekend. To that end, a seriously revised MP4-27 has been brought here this weekend.

Bianchi is in di Resta's Force India, while Bottas replaces Senna and Clos is in Karthikeyan's HRT.

Race Control reveals that a "light shower" is expected between 10:05 and 10:10, as Vergne heads down the pitlane to get the weekend underway. He is followed by Ricciardo, Kovalainen, Glock, Grosjean, Perez, Raikkonen and Uncle Tom Cobbly.

Just three minutes into the session all but Massa and Schumacher have been out. Two bright lights shine from underneath the McLaren, laser sensors as the Woking outfit ensures that every last drop of data is gleaned.

Its becoming ever more common for teams to run sensors, however, McLaren's are the most exciting we've seen to date.

Kobayashi is the first driver to post a time, the Sauber driver crossing the line at 1:55.569.

In the McLaren garage mechanics swarm over Hamilton's car checking absolutely everything, one crew member is even taking pictures.

Perez posts 23.198 and Kovalainen 25.851 before Button gets serious with a 17.410.

Seventeen minutes into the session, and with the rain failing to appear, more and more drivers come out to play. Button raises the bar with a 17.240 as Grosjean posts 18.482 to go second.

A big lock-up form Webber and Kovalainen goes cross country as the first spots of rain finally start to fall. In the stands fan don their jackets and umbrellas are raised.

Vettel warns that it is quite slippery in Turns six and eight as Race Control declares the session 'wet'.

Despite the best efforts of the cameraman, he is unable to break the wall of 'minders' shielding Raikkonen's car from scrutiny. The Enstone outfit resorting to Red Bull levels of secrecy and paranoia.

As the rain falls, albeit lightly, only the Toro Rosso duo are on track, both on the mediums. Thus far there's been a fair amount of running on the experimental hard.

A few minutes later the rain stops as quickly as it began, and in no time at all Hamilton, Massa, de la Rosa, and Perez join the Toro Rossos.

As Massa goes fifth (19.781) on the experimental rubber and Rosberg seventh (20.860) it starts to rain again. This time it's a bit harder, consequently most drivers return to the pits. Kobayashi has a major off-track excursion, just in case anyone wasn't convinced by the conditions.

Massa stays out for a couple of laps but then he too decides enough is enough. As the Brazilian returns to the pits his place is taken by Bottas and Pic.

At half-time, it's: Button, Grosjean, Perez, Bottas, Massa, Vergne, Rosberg, Kovalainen, Hamilton and Glock. Ricciardo is eleventh, ahead of Petrov, Webber, Clos, Bianchi, de la Rosa, Pic and Kobayashi. Raikkonen, Maldonado, Hulkenberg, Vettel, Alonso and Schumacher have yet to post a time.

The rain stops (again) leading to a flurry of drivers heading down the pitlane, among them Raikkonen is the super-secret Lotus. Elsewhere, Vergne does brilliantly to catch the Toro Rosso as he runs wide over the kerb in Turn 1.

As Raikkonen heads back to the pits, having completed an exploratory lap, Schumacher advises his pit crew that there's a rain shower on the way.

A 17.795 sees Alonso go second, Button's place at the top of the timesheets finally under pressure. Moments later, Hamilton goes second with a 17.926.

Following a brief off-track moment, Schumacher posts 18.247 to go fifth only to be demoted when his teammate posts 17.915. Moments later, Hulkenberg goes second overall with a 17.599.