yeah, it really depends, the condition of the playing field and to what extent it's going to be affected by the weather is an issue too, it also depends on whether the players are accustomed to the weather conditions, for example an italian or a greek team will be much more influenced by a cold weather than say a norwegian team. It's rumored that it's a big leveler in terms of the underdog and the favourite, but that is open to question as well. Some people also claim that the more tecnical team is affected, and that is often true but open to questions, take for example the cl game with chelsea and barcelona where game 1 was played in a dump because besides the weather chelsea had been watering the field for the very reasons mentioned above, but what really happened was instead of barca having more problems because of the technical prowess, exactly because they are a technical team, they werer affected less. All that said I don't know how much weather affects american football that nylon mentioned in terms of slowing teams down, but it does affect soccer too this way, more towards the under than anything if one had to choose. Let's face it players are not machines and extreme weather conditions means their stamina and endurance isn't as effective. Dunno if I 've been of any help, this issue is kinda, well, murky.