9.1: Streamlines and trajectories aren’t usually the same.
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Streamlines are lines that are everywhere parallel to the velocity vectors at a fixed time. They consider the direction of the velocity but not the speed. Sometimes more streamlines are drawn to indicate greater speed, but this is not usually done. Streamlines generally change from one time to the next, giving us “snapshots” of the motion of air parcels. For maps of wind observations for a fixed time, we often look at streamlines. On a map of streamlines, you will see that the lines aren’t always straight and don’t always have the same spacing. Confluence is when streamlines come together. Diffluence is when they move apart.
Trajectories are the actual paths of the moving air parcels, and indicate both the direction and velocity of air parcels over time. Convergence is when the velocity of the air slows down in the direction of the streamline. Divergence is when the velocity of the air speeds up in the direction of the streamline. We will talk more about convergence/divergence later, but for now, you should understand that convergence/divergence come from changes in velocity while confluence/diffluence come from changes in spacing between streamlines.
NOAA HYSPLIT model forecast of wind trajectories for June 13, 2015. The top figure is the horizontal view; the bottom figure shows the vertical motion of the trajectories. The distance between squares on individual trajectories indicates 6 hours of travel time. When the squares on a trajectory get closer together with time, there is convergence. When the squares get further apart with time, there is divergence. Credit: NOAA ARL
Confluence/diffluence and convergence/divergence are illustrated in the figure below: