•It is important to distinguish
clearly between streamlines, which give a “snapshot” of the velocity
field at any instant, and trajectories, which trace the motion
of individual fluid parcels over a finite time interval.
•The geopotential height contour
on synoptic weather maps are streamlines not
trajectories.
•In the gradient balance, the
curvature (R) is supposed to be the estimated from the trajectory,
but we estimate from the streamlines from the weather maps.