Why is The Four Seasons considered an early example of program music?

The Four Seasons by Antonio Vivaldi is considered an early and detailed example of program music—instrumental music intended to evoke a narrative, scene, or idea—because of its explicit connection to descriptive text.1

Unusually for the early 18th century, Vivaldi published these four concerti alongside accompanying sonnets that elucidated the specific imagery the music was meant to portray. Vivaldi took great care to translate these poetic lines directly into the musical score. For instance, he used specific instrumental techniques to mimic natural occurrences described in the poems, such as the barking of a dog in the viola section during the Spring concerto or the sounds of storms and bird calls throughout the set. By linking the structure of his music to the three-part narrative of the poems, Vivaldi created a work where the music serves as a direct expression of the provided literary program.1