What does FWHM reveal about a velocity distribution?

[Carnot]

Hi

FWHM on a velocity distribution provides me with a specific velocity. What does FWHM say about the velocity distribution, I mean, does FWHM give me the most probable velocity of the distribution or something like that?

Thanks

 

[Simon Bridge]

FWHM means "full width half maximum" - it is not a velocity, but a range of velocities.
It is a measure of the spread of a distribution...
http://en.wikipedia.org/wiki/Full_width_at_half_maximum

The most probable velocity is usually the mean, like you'd expect.

 

[Vanadium 50]

The most probable velocity is usually the mean, like you'd expect.

No, that's the mode. It might or might not be anywhere near the mean.

 

[Simon Bridge]

Depends on the distribution aye - I'm used to FWHM being used for pretty symmetrical distributions though.

 

[Vanadium 50]

Consider the Maxwell-Boltzmann distribution - surely a common (if not the most common) velocity distribution. It's not all that symmetrical - there's a long tail to the right, and thus the average and most probable velocities differ by a factor of 2/sqrt(pi).