Answer: The air is cool and still in the morning.
Explanation: Prepositional phrases are characterized as the phrases comprising of a preposition and its object or compliment. These phrases primarily function as an adjunct or as a modifier in order to add detail to the sentence. In the above sentence, the phrase 'in the morning' is the prepositional phrase with the preposition 'in', its object 'morning,' and 'the' as the modifier of the object. It is functioning as the adverbial phrase which modifies the adjective 'cool and still.'