What units is the 'Normal' in the 'Initial Velocity' in the Fluid Sim smoke source settings?

I suspected it wasn’t m/s, and did a quick check:

I set up a simple circle mesh emitter facing downwards and a domain with buoyancy density and heat set to zero in the smoke settings so the smoke didn’t go anywhere

With the circle emitter ‘initial velocity’ → ‘Initial Z’ set to -1 m/s:


The smoke took 100 frames to reach the bottom of the domain

And now the same setup but with ‘Normal’ set to 1 (and z set back to 0):


The smoke hasn’t been expelled half as forcefully.

Also, I think the tooltip on the ‘Surface Emission’ setting is incorrect, or at least seems misleading:

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This to me sounds like it offsets the emission from the surface, when in reality, this controls the amount of emission from the surface

By contrast the tooltip for volume emission is worded correctly:
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Hi, you need to set the velocity Z, to - or + not the surface emission.

I don’t think you’ve fully understood me, what I’m saying is that setting ‘Normal’ velocity on a plane which is facing upwards to 1 is not the same as setting Z velocity to 1m/s.

(The tooltip of the ‘surface emission’ setting is an entirely different thing)