A floating city above the surface of Ardana, home to the elite who rule over the planet's Troglyte miners.
Stratos is a city held in the clouds above Ardana by antigravity. It is a place of art and leisure, home to the council and their families, while the work of the planet is done by the Troglyte miners below.
In The Cloud Minders, Kirk and Spock visit Stratos seeking zenite to stop a plague, and find its inhabitants fearful of rebellion. The city's isolation from the gas-filled mines helps explain why its people seem more intelligent than the miners.