Comment Re:Superman... (Score 2) 116
'Per unit volume' is used all the time in cases where the exact unit doesn't matter, other than it being volumetric. 15,000 raindrops per cubic meter, 120 raindrops per cup, etc. Outside of my field, I think the most I've ever seen that term used is to define density: "Mass per unit volume".
As far as the first point goes: Try envisioning a horizontal rectangle with a constant amount of dots in it, where the dots move downward from the top at a constant rate at a random point along the x-axis, with a bar sweeping across the rectangle. Over repeated trials, the bar will touch the same amount of dots regardless of how fast it sweeps across the rectangle, but the odds of a dot spawning right on the bar (rain landing on your head) is decreased at higher speeds.
How well this holds up in non-theoretical cases is a different story, like rain coming in at an angle while hunched? Which side of you getting wet is a consideration too.