We recognize the second word as the source for the English word "martyr," and we remember how the idea of being a witness to Christ became identified with the idea of being killed for one's faith in Christ (i.e., by correlation). That's the kind of faith God approves: not only is such a witness willing to shed blood, he is quite likely to be given the opportunity. (Little wonder two or three out of five family members aren't interested in it.)
This is the only appearance of the word nephos in the New Testament. Every other cloud is a nephele. I'm told that "nephele" denotes a single, distinct, bounded cloud, while "nephos" denotes a mass of cloud that fills the sky, which is why the NAB (following Douay-Rheims) translates the latter, not merely as "cloud," but "so great a cloud."
For practical purposes, then, the witnesses of faith who surround us are innumerable. They extend back in time, to the second son of Adam at the earliest, and out in all directions. They testify to faith in Christ in both word and deed. Catholics rightly think of this cloud as being around us in a particular way during Mass, but we would be wrong to think of them as not being around us always.