Ahead of this morning's opening practice session, the air temperature is 22 degrees C, while the track temperature is 22 degrees. Though it was warm and sunny earlier, around forty minutes ago there was a brief shower, then another one, then another one. All in all, it's typical Spa.

Other than the weather, which is always a talking point here, the big news is that Bruno Senna replaces Nick Heidfeld at Renault, though the German is taking legal action to ensure he's back in the seat as soon as possible.

Ignoring Quick Nick's situation, what will be interesting to watch is how Senna deal with things, his recruitment a reminder of when Romain Grosjean was drafted in at short notice to replace Nelson Piquet.

Only one 'Friday driver' is making an appearance this morning, Karun Chandhok taking over the wheel of Heikki Kovalainen's Lotus.

While the ever changing weather is sure to dominate the headlines over the course of the weekend, so to is the fact that Michael Schumacher is celebrating the twentieth anniversary of his F1 debut - appropriately at this very circuit. Indeed, Spa has played such a large part in the Schumi Story, the German refers to the majestic Ardennes track as his 'living room'.

Kobayashi leads way when the lights go green, however, the cameras cuts to Heidfeld who is chatting in the paddock, still wearing his Renault team gear.

Already there are 15 drivers on track, including local hero d'Ambrosio and Schumacher, who is sporting a special golden helmet.

Four minutes into the session all but Hamilton have been out, Kobayashi, Rosberg and Schumacher remaining on track for a second lap. Appropriately, the seven-time champ posts the first time of the weekend (1:55.007) while Rosberg posts 57.926.

With the sun now shining the track appears nice and dry - though that doesn't stop Schumacher getting beautifully out of shape - however, Lotus warns of rain in five minutes.

The Mercedes duo get down to 54.355 (Schumacher) and 54.829 (Rosberg) as Trulli, di Resta and Sutil come out again. At which point it starts to rain.

Ferrari, which is testing some new aerodynamic components, appears to be having problems with its new 'windscreen', Massa complaining that the air flow over it is making his helmet heavy. He is clearly not impressed when the team sends him out for a second lap without appearing to pay attention to his problem.

The rain increases in intensity, Mike Gascoyne conceding that it is now too wet for dry tyres. Indeed, it is too wet for rain tyres, it's really lashing down.

Other than a revised driver line-up, Renault has brought a new floor and numerous aero updates.

As quickly as it began, the rain stops, indeed, the sun begins to shine, at least in the pitlane. On the other side of the track however, it's still raining. Changeable conditions over the course of a session or race is one thing, but changeable conditions over the course of a lap quite another.

Thirty-three minutes into the session, following fifteen minutes of total inactivity, Vettel heads down the pitlane, the German having taken his first F1 win in similar conditions, albeit at Monza.

"Take it easy," the German is told, "you can probably see there are some rivers." Having seen the conditions first hand, the world champion returns to the pits.

No sooner has Vettel entered his garage than Hamilton leaves his, McLaren having urged its drivers to be aggressive this weekend. Interestingly, the Englishman is on inters, whereas Vettel was on full wets. The rain has stopped but the track remains treacherous, a mighty wobble as he aquaplanes in Blanchimont proving the point.

Hamilton returns to the pits, the cameras picking up the fact that he and his teammate are running completely different rear wing set-ups. Speaking earlier in the week, team MD Jonathan Neale claimed that the new rear wing the team is bringing here would make a big difference.

Sutil posts the third time of the day, the Force India driver crossing the line at 2:16.282.

As more and more drivers pour on to the track - no pun intender - the inconsistent conditions are causing problems. Some opt for inters others stick with full wets.

Fifty minutes into the session, Alguersuari goes third (2:12.502), ahead of Buemi, Sutil, Senna, Kobayashi and Perez.