Final answer:
In the given sentence, 'lucid' describes the air as clear or transparent, which allows birds to gracefully move through it.
Step-by-step explanation:
The word lucid in the sentence "Birds dipped their wings in the lucid flow of air" can be understood as clear, bright or transparent.
This adjective describes the air in a way that metaphorically suggests it is so clear and pure that it allows the birds to move through it without obstruction, giving a vivid image of freedom and elegance in their flight. Essentially, lucid in this context conveys the transparency and purity of the air, enabling the birds' graceful motion.