Fundamentals of Atmospheric Science

9.1 Streamlines and trajectories aren’t usually the same.

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.

Wind map of wind streamlines over the continental United States as described in the caption and text above
Wind map of wind streamlines over the continental United States. You can see many different examples of confluence (streamlines coming together) and diffluence (streamlines moving apart). See the wind map in motion as a series of streamline maps.
Credit: Creative Commons via Fernanada Viegas and Marten Wattenberg

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 as described in the caption
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:

Confluence / diffluence and convergence / divergence illustration as described in the text
Examples of confluence/diffluence and convergence/divergence.
Credit: H.N. Shirer

Extra Credit Reminder!

Here is another chance to earn one point of extra credit: Picture of the Week!

  1. You take a picture of some atmospheric phenomenon - a cloud, wind-blown dust, precipitation, haze, winds blowing different directions - anything that strikes you as interesting.
  2. Add a short description of the processes that you think are causing your observation. A Word file is a good format for submission.
  3. Use your name as the name of the file. Upload it to the Picture of the Week Dropbox in this week's lesson module. To be eligible for the week, your picture must be submitted by 23:59 UT on Sunday of each week.
  4. I will be the sole judge of the weekly winners. A student can win up to three times.
  5. There will be a Picture of the Week dropbox each week through Lesson 11. Keep submitting entries!