Formula One's summer break came to a shuddering halt as heavy rain and leaden skies welcomed back the teams and drivers.

The Belgian Grand Prix is renowned for the capricious nature of its weather, and that was certainly highlighted throughout the opening practice session ahead of Sunday's race.

The gloomy forecast for a sodden day was certainly correct as the rain barely relented from the moment the drivers arrived at the track almost through to the end of the 90-minute run.

Only for the final ten minutes did the skies become brighter, and that was when Fernando Alonso set what proved to be the quickest time of the session.

Alonso was already leading the way anyway, but improved to a time a fraction over two minutes of 2:00.727 to finish 0.770secs ahead of Lewis Hamilton in his McLaren.

Renault's Robert Kubica, who had posted only one installation lap for around the opening 70 minutes, eventually emerged to put in a solid run towards the end and finish with the third best time of 2:02.081.

With further rain forecast for the weekend, there was at least a point to be made for all the drivers to get some running under their belts.

Red Bull's Sebastian Vettel was fourth, followed by the Force India of Adrian Sutil and reigning world champion Jenson Button in his McLaren, but just over two seconds down on Alonso.

The top ten was completed by Red Bull's championship leader Mark Webber on his 34th birthday, Sauber's Kamui Kobayashi, Rubens Barrichello in his Grove-built Williams and on the occasion of his 300th grand prix, and then Michael Schumacher for Mercedes.