people out enjoying the cool evening air
Refers to people who go outside on a hot evening to enjoy the cooler air, often in a park, by a river, or on a veranda.
夕方になると、川沿いに涼み客が増える。
In the evening, more people come out to enjoy the cool air along the river.
縁側で涼み客が団扇を使っている。
People cooling off on the veranda are using round fans.
Compound of 涼み (noun form of verb 涼む 'to cool off') and 客 ('guest, visitor').