Longitudinal modes sometimes occur as counter-rotating vortices (hairpin vortices) and sometimes are associated with streaks of low momentum. “Longitudinal” refers to the fact that the axes of the vortices are aligned with the wind vector.  Longitudinal modes are common in numerical simulations but 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 are moving perpendicular to the wind.

Lidar images show what might be either a field of organized longitudinal modes or possibly wave modes.  The appearance of wave motions and longitudinal structures can be similar and the distinction in the terminology generally refers to the instability mechanism leading to their development.  In these examples, the structures were somewhat aligned with the wind, which is characteristic of longitudinal instabilities.