Dogs do see better than we do in dim light but remember that's a generalization that compares all dogs to all humans. Individual dogs may not see as well as their handler in dimly lit conditions. Indoor lighting can present odd shadows that while confusing to the dog may not be noticed by the handler. A bright red jump set against a background of institutional green may be very hard for the dog to see and correctly judge. Other color combinations may prove equally difficult.
Environment is fairly self explanatory.
Cold, wet weather is hard on joints and muscles, making them more susceptible to injury. Even with appropriate warmup, muscles can chill very quickly keep in mind that even in the heat of August, major league baseball pitchers are brought to the mound with a jacket thrown over those valuable muscles. In cold weather, even what seems a brief wait at the start line or in-gate can result in chilled muscles. Hot humid weather presents a serious threat. Unable to sweat and thus cool their bodies, dogs depend on evaporation over their tongue for heat exchange.