Rain marred the first few hours of running, but at least offered a chance to sample the wet and intermediate tyres.
A brief period of respite in the weather then allowed most teams to put in a reasonable number of slick-shod laps before the next shower shortly before the lunch break.
Hamilton and Mercedes were first to give slicks a go and were soon on top.
There followed a frantic period of improvements as teams experimented with how various compounds of slick tyre behaved on a drying track.
Felipe Massa, Jules Bianchi and Jenson Button all appeared at the head of the order, before Hamilton reclaimed the top spot with a 1m23.282s on medium Pirellis.
Button completed the morning second for McLaren, 0.351 seconds behind, followed by Jean-Eric Vergne's Toro Rosso and Bianchi's Force India.
There were plenty of red flags and incidents when the weather was at its worst.
Massa slid into the gravel at Turn 4 just 10 minutes into the morning, Giedo van der Garde put his Caterham off at the final corner, and both Sauber's Esteban Gutierrez and Vergne had similar incidents at Turn 5.
Webber parked his Red Bull with an issue just as Gutierrez caused his stoppage, and van der Garde prompted a fifth red flag when he stopped after Campsa just before lunch.
Valtteri Bottas did not set a flying lap time as Williams was focused on pitstop practice, but did complete 23 in/out-laps.
1. Lewis Hamilton Mercedes 1m23.282s 44
2. Jenson Button McLaren 1m23.633s +0.351 31
3. Jean-Eric Vergne Toro Rosso 1m24.071s +0.789 37
4. Esteban Gutierrez Sauber 1m25.239s +1.957 24
5. Jules Bianchi Force India 1m25.732s +2.450 51
6. Giedo van der Garde Caterham 1m27.429s +4.147 23
7. Felipe Massa Ferrari 1m27.563s +4.281 47
8. Mark Webber Red Bull 1m27.616s +4.334 34
9. Max Chilton Marussia 1m29.902s +6.620 21
10. Romain Grosjean Lotus 1m34.800s +11.518 19
11. Valtteri Bottas Williams no time 23