Atmosphere changes on powder "throws"

[woodnut]

This may have been covered before, but I did not see it, so I'm asking ....
I have noticed that from day to day, two things occur in my loading setup. One, the scales will vary; one day the empty (no pan on it) weight will be .35 and on day two, .30 or even .25 or .20. Two, as I'm using the Harrell measure each day and leave it at the same setting, the charge wt. will vary,too. So, I get the scale to settle on a constant empty wt. and throw "the days" powder wt. as the same as the day (s) b4, or another way to say the same ,say, 41.0 grains. But I have to wonder, is it the same wt.?
So, is it more important to have the same volume or wt? Is the change from one day to the next significant? If it's volume, why weigh each day if you keep the same powder measure setting? I remember benchresters (from a long time ago) said volume.
What's your thoughts?

 

[MagnumManiac]

Every factory that makes ammunition loads by VOLUME. This is why factory ammo varies in weight a little between rounds.
I also load by volume. Atmospheric conditions can change the weight of the powder, so I trust the setting on my measure.
I do check against a scale periodically, but only concern myself with huge variations over a grain of powder discrepancies between settings.

Cheers.

 

[woodnut]

Basically, what I'm doing. Thank you MagnumManiac.