In act 1, it is shown to be a very hot weather with all 12 jurors sitting in an un air conditioned room. This begins to become very irritable for many jurors such as juror 7 mopping his brow and saying "y'know something? i phoned up for the weather. this is the hottest day of the year. And the foreman inspecting the fan, very desperately trying to get it fix. By having the hot weather, it allows tension to increase and also creates the scene to become interesting. The heat relates greatly and makes it harder on the jurors since they need to choose sides if the kid is "GUILTY" or "NOT GUILTY"
Also when it begins to rain, the jurors start to become irritated and more fustrated and angry, then when it storms it shows big tension and big fights happening. This become a large dramatic effect and allows it to related to the play.
in this video : http://www.youtube.com/watch?v=JdzFtOEkhYQ&feature=related
In this scene of the video, the heat is affecting all jurors by allowing tension to raise to a higher level and also give them a demanding reason to argue and debate with each other.