Ahead of today's second practice session, the air temperature is 23 degrees C, while the track temperature is 30 degrees. Although bright there is a lot more cloud than this morning, with more than a hint of moisture in the air. Indeed, in the distance, the other side of the mountains that surround the Principality, there have been a couple of lightning bolts.

Fernando Alonso topped the timesheets earlier, ahead of Grosjean, Perez, Hamilton, Maldonado and Massa, all covered by just 0.578s.

The session was cut short when, five minutes before the end, Kovalainen's engine detonated in the tunnel, leaving a thick cloud of smoke and a trail of oil, a somewhat frightening combination considering the location.

Having completed an installation lap, Kimi Raikkonen declared himself unhappy with the car, and with the team making numerous changes to its front, mostly in terms of steering, the Finn never made another appearance. That said, the pace of his teammate must leave him optimistic.

Due to the changing condition we may well witness a flurry of activity in the opening phase of the session. However, with rain forecast on Saturday and Sunday we could well see plenty of running should conditions really deteriorate.

The lights have barely gone green when Kobayashi, heading the queue at the end of the pitlane, disappears up the hill. He is followed by Perez, Hulkenberg, di Resta, Vergne, Glock, Petrov and Raikkonen. Indeed, within moments there are 16 drivers on track.

As all but Alonso and Kovalainen take to the track, Kobayashi posts 20.617 but this is soon beaten by his teammate who crosses the line at 18.800. Moments later, di Resta splits the Saubers with a 17.732.

As Alonso heads down the pitlane, leaving Kovalainen as the only 'stay at home', Kobayashi retakes the top spot with a 17.504. However, the Japanese is subsequently eclipsed by Hamilton, Schumacher and then Button (17.095).

Perez, now down in ninth, goes quickest in S1 as Alonso goes quickest overall (16.944). The Mexican fails to improve as heavy traffic in the final sector costs him precious time.

Barcelona winner Maldonado posts 16.820 to take the top spot, his teammate currently tenth, just ahead of Massa and Vettel.

A 16.661 sees Alonso retake the top spot with Grosjean taking second. Moments later, Button, the first driver to try the option (supersoft) tyres posts a 15.746, almost a second quicker than Alonso's best.

Grosjean (on primes) posts 16.138 to close to within 0.392s of the McLaren.

More drivers switch to the options as the rain clouds close in, among them Maldonado, di Resta, Perez, Petrov and Vergne.

With 1:14.40 on the clock the stewards declare the track to be wet, though to the naked eye there only appear to be a few drops. As if to confirm the decision, the fans move to the covered part of the grandstands and the drivers return to the pits.

Kovalainen, who has completed just 4 laps, is told that there will be "twenty minutes of rain". Then, after a short dry period there will be a heavier downfall.

At this stage, with 1:05.15 in the clock, it's: Button, Grosjean, Massa, Alonso, Maldonado, Rosberg, Webber, Kobayashi, Schumacher and Vettel. Of those, Button, Maldonado, Webber and Kobayashi have tried the option rubber.

With 55.30 on the clock, as Petrov insists that he doesn't want to be the first out, Timo Glock heads down the pitlane. A few moments later he is followed by Alonso. Raikkonen also opts to come out, the Finn having missed out this morning. With it not being wet enough for inters, all three are on the prime tyres.

As more drivers take to the track, aware that the rain will get heavier, Hulkenberg and Raikkonen both posts PBs in S1. The German is on options, as are several other drivers.

As drivers appear to ease off, and others return to the pits, its clear that there is rain falling somewhere out there. They are particularly cautious at Mirabeau and the Hairpin.

With 44:19 on the clock, everyone is back in the pits, silence descends on the Principality.

At 33:50, the Toro Rossos head down the pitlane. A few minutes later they are joined by the Marussias. All four are on inters.