McLaren dominated a rain-hit opening practice ahead of the German Grand Prix. Jenson Button topped the time sheest followed by team mate Lewis Hamilton.Intermittent showers made conditions tricky for drivers although the rain was nothing on the scale which caused chaos at the British Grand Prix at Silverstone.Valtteri Bottas crashed his Williams near the end of the session, smashing into the barriers on corner 13 and emerging unscathed to survey the damage.WEATHER WOESButton set the best time of one minute 16.595 seconds early in the session before the rain began while Hamilton, who will be taking part in his 100th Grand Prix on July 22, was 0.498 seconds behind.They were followed by current championship leader Ferrari's Fernando Alonso and Mercedes' Michael Schumacher. Conditions meant it was difficult to read anything significant into the timings.The German Grand Prix is seen as crucial to McLaren's season after poor performances in the previous two races saw them slip down the standings.The session was the debut for the new version of the cars following a series of upgrades.The Red Bull pair made little impact with Sebastian Vettel 12th fastest and Mark Webber 20th. First practice session results from the German Grand Prix:1. Jenson Button (Britain) McLaren 1min16.595 2. Lewis Hamilton (Britain) McLaren 1min17.093 3. Fernando Alonso (Spain) Ferrari 1min17.370 4. Michael Schumacher (Germany) Mercedes 1min17.382 5. Sergio Perez (Mexico) Sauber 1min17.413 6. Nico Huelkenberg (Germany) Force India 1min17.599 7. Nico Rosberg (Germany) Mercedes 1min17.915 8. Felipe Massa (Brazil) Ferrari 1min17.995 9. Pastor Maldonado (Venezuela) Williams 1min18.020 10. Romain Grosjean (France) Lotus 1min18.130 11. Kamui Kobayashi (Japan) Sauber 1min18.226 12. Sebastian Vettel (Germany) RedBull 1min18.339 13. Valtteri Bottas (Finland) Williams 1min18.422 14. Daniel Ricciardo (Australia) Toro Rosso 1min18.709 15. Kimi Raikkonen (Finland) Lotus - Renault 1min18.831 16. Jules Bianchi (France) Force India 1min18.972 17. Jean-Eric Vergne (France) Toro Rosso 1min19.039 18. Vitaly Petrov (Russia) Caterham 1min19.674 19. Heikki Kovalainen (Finland) Caterham 1min19.963 20. Mark Webber (Australia) RedBull 1min20.122 21. Charles Pic (France) Marussia 1min20.169 22. Timo Glock (Germany) Marussia 1min20.539 23. Pedro de la Rosa (Spain) HRT 1min21.138 24. Dani Clos (Spain) HRT 1min21.740Stay with Wheels24 for the German Grand Prix Formula 1 weekend!