Mark Webber quickest for Red Bull between showers in Spa practice

Only the two Mercedes had been able to squeeze in flying laps on slicks in first practice before the rain arrived, and the track was damp again when the afternoon session got underway.

Conditions steadily improved, with the two Red Bulls, Fernando Alonso and Jenson Button all taking turns at the front before Webber set the 2m00.964s that would stand at the best intermediate tyre lap of the session.

Alonso then became the first to explore the circuit on slicks, and the Ferrari man immediately beat Webber's time by four seconds, before getting down to a 1m53s on the next lap.

Both Mercedes and both Red Bulls would have moments in first position during the frantic quarter of an hour that followed as everyone tried to make the most of what would prove to be the only dry segment of the session.

It was Webber who was on top on a 1m50.321s when the clouds opened once more with just under 40 minutes left, and although most drivers did eventually come back out to get some wet practice, there was no way anyone was going to improve their pace on what was soon a sodden circuit.

Alonso ended up second, ahead of McLaren pair Jenson Button and Lewis Hamilton, and the second Ferrari of Felipe Massa.

Behind Nico Rosberg (Mercedes) and Sergio Perez (Sauber), Nico Hulkenberg was eighth in a rare Friday afternoon appearance for a reserve driver - Force India having hoped the morning would see the best of the weather. It was Adrian Sutil who stood aside for Hulkenberg, with Paul di Resta ninth in the repaired sister car, ahead of Vettel.

Morning pacesetter Michael Schumacher was 11th for Mercedes, while a steering problem left Renault's Vitaly Petrov at the back of the field.

Pos  Driver              Team/Car              Time       Gap       Laps
 1.  Mark Webber         Red Bull-Renault      1m50.321s            22
 2.  Fernando Alonso     Ferrari               1m50.461s  + 0.140s  18
 3.  Jenson Button       McLaren-Mercedes      1m50.770s  + 0.449s  9
 4.  Lewis Hamilton      McLaren-Mercedes      1m50.838s  + 0.517s  9
 5.  Felipe Massa        Ferrari               1m51.218s  + 0.897s  14
 6.  Nico Rosberg        Mercedes              1m51.242s  + 0.921s  22
 7.  Sergio Perez        Sauber-Ferrari        1m51.655s  + 1.334s  20
 8.  Nico Hulkenberg     Force India-Mercedes  1m51.725s  + 1.404s  17
 9.  Paul di Resta       Force India-Mercedes  1m51.751s  + 1.430s  8
10.  Sebastian Vettel    Red Bull-Renault      1m51.790s  + 1.469s  13
11.  Michael Schumacher  Mercedes              1m51.922s  + 1.601s  22
12.  Pastor Maldonado    Williams-Cosworth     1m52.750s  + 2.429s  20
13.  Kamui Kobayashi     Sauber-Ferrari        1m52.780s  + 2.459s  25
14.  Jaime Alguersuari   Toro Rosso-Ferrari    1m52.911s  + 2.59 s  24
15.  Sébastien Buemi     Toro Rosso-Ferrari    1m53.009s  + 2.688s  24
16.  Rubens Barrichello  Williams-Cosworth     1m53.156s  + 2.835s  17
17.  Bruno Senna         Renault               1m53.835s  + 3.514s  20
18.  Jarno Trulli        Lotus-Renault         1m55.051s  + 4.730s  20
19.  Timo Glock          Virgin-Cosworth       1m55.494s  + 5.173s  22
20.  Heikki Kovalainen   Lotus-Renault         1m56.202s  + 5.881s  15
21.  Jerome D'Ambrosio   Virgin-Cosworth       1m56.816s  + 6.495s  20
22.  Tonio Liuzzi        HRT-Cosworth          1m57.450s  + 7.129s  19
23.  Daniel Ricciardo    HRT-Cosworth          1m57.612s  + 7.291s  24
24.  Vitaly Petrov       Renault               2m02.234s  + 11.913s 12

All timing unofficial