While grilling, the lid should be kept closed. This allows the grill to maintain a consistent circulation of heat, and it allows the food to retain delicious flavors.
We've got a saying here at Weber..."Lookin' Ain't Cookin'".
Cooking with the lid closed is one of the hallmarks of a Weber grill, and it's what set our grills apart from the competition back in 1952, when George Stephen made the first Weber grill.
For some Weber history lessons check out these stories.