A cloudless night with low wind speed indicates:

A cloudless night with light wind lets the ground lose heat rapidly by radiation, cooling the lower air and giving a stable atmosphere.

A turbulent atmosphere requires strong mixing and gusty winds, which are absent on a calm night.

An unstable atmosphere arises when the surface is strongly heated so warm air rises, the opposite of night-time cooling.

A neutral atmosphere occurs under moderate winds and cloud, not under clear calm conditions.

Radiative cooling forms a temperature inversion near the ground that suppresses vertical motion.

Such stable inversions trap pollutants close to the surface, worsening night-time air quality.