On clear nights, outgoing radiation leads to cooling of the surface, which cools the air adjacent to the surface.  The temperature closest to the ground surface is the coldest. The increase of (potential) temperature with height corresponds to stable stratification.  Stable stratification acts to suppress the turbulence.  At night the turbulence is generated only by wind shear whereas in the daytime, the turbulence is generated by both surface heating and wind shear.  With weak winds and clear skies, the turbulence is particular weak, and may not be detectable by a human observer without sensitive instrumentation. With weak winds, such turbulence is often generated by submeso motions and local circulations attached to the surface.

We now outline different types of turbulent structures in the nocturnal boundary layer with the recognition that any type of classification scheme is idealized and incomplete.

a) longitudinal modes – SUB MENU

Longitudinal modes sometimes occur as counter-rotating vortices (hairpin vortices) and sometimes are associated with streaks of low momentum that are common in numerical simulations.  “Longitudinal” refers to the fact that the axis of the vortices are aligned with the wind vector.  Longitudinal modes are uncommon in our fog videos.  However, before conclusions can be made,  the fog release strategy must be revised to include truck-mounted releases that are more elevated and moving perpendicular to the wind.

b) transverse overturning – SUB MENU

The axes of transverse vortices are perpendicular to the wind and commonly observed in our fog videos.   Such vortices transport warmer air downward and lift colder air upward.

c) Vertically oriented vortex motions. – SUB MENU

Vortex motions in the vertical (vertical vorciity) includes dust devils but also includes much more common weaker vortex motions in both the stable (clear nights) and unstable (daytime surface heating) boundary layers.

d) fine scale and striations – SUB MENU

Fine-scale turbulence can be visualized in terms of slow diffusion of thin striations in the fog videos and seems to occur in cases with very weak winds < 1 m s^{-1} and clear nights that frequent some of our sites.

e) Turbulent patches – SUB MENU

Patches of turbulence generally refer to a small region of small eddies often separated by larger regions of very weak turbulence such as fine-scale turbulence. Downward bursting of turbulence is generated at higher levels and can lead to patches of turbulence near the surface.  This downward busting brings warmer air and higher momentum to the surface.