During what time does the stable boundary layer typically occur?

The stable boundary layer typically occurs at nighttime, particularly when stability classes D through F are present. During the night, the surface cools, leading to a temperature inversion where warmer air rests above cooler air near the ground. This temperature gradient creates stability, which inhibits vertical mixing and can trap pollutants close to the surface.

Stability classes D through F are associated with conditions that contribute to a stable atmosphere. Specifically, class D represents moderately stable conditions, while classes E and F indicate increasingly stable environments. In such stable layers, the dispersion of airborne substances is more limited compared to unstable conditions.

In contrast, during the daytime, especially with stability classes A through C, the atmosphere tends to be more turbulent and allows for better mixing of air, which enhances dispersion. Thus, the very nature of the stable boundary layer is reflected in its occurrence during nighttime under conditions that restrict vertical movement, making the choice of nighttime with stability classes D through F the correct context for this phenomenon.