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.
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.
Confluence/diffluence and convergence/divergence are illustrated in the figure below:
Extra Credit Reminder!
Here is another chance to earn one point of extra credit: Picture of the Week!
- 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.
- Add a short description of the processes that you think are causing your observation. A Word file is a good format for submission.
- 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.
- I will be the sole judge of the weekly winners. A student can win up to three times.
- There will be a Picture of the Week dropbox each week through Lesson 11. Keep submitting entries!