Characteristics

 (1) Streamlines are nearly parallel
 (2) Streamlines are nearly stable (at a point the wind velocity
       changes little in speed or direction with time)

Tends to occur when

 (1) Wind velocity is low
 (2) Surface is smooth and free of obstacles
 (3) Surface is uniform in temperature and comparable to or
       cooler than the air above

Characteristics

 (1) Streamlines are non-parallel
 (2) Streamlines are unstable (at a point the wind velocity
       fluctuates in speed and direction with time) (wind is gusty)

Tends to occur when

 (1) Wind velocity is high
 (2) Surface is rough
 (3) Obstacles are present
 (4) Surface is non-uniform in temperature or much warmer than
       the air above ("thermal turbulence")