MIKE MOSS SAYS: Donnie, I would have thought they smell better right after a bath -- just kidding! The sense of smell is modulated somewhat by environmental factors, including pressure, humidity and temperature, but it is tricky to separate a direct effect on the sensitivity of olfactory nerves, for example, from the effects these same factors can have on the particulates and gasses that are being detected by the nose.
There does seem to be evidence that the sense of smell is better under higher pressure rather than lower, at warmer rather than colder temperatures, and at higher rather than lower humidity levels. Whether a pressure of 30.00 inches of mercury is a common threshold for noticing an improvement in hunting dog sensitivity, I don't know, but 30 inches is at least a little ways on the high side of "standard" sea level pressure of 29.92.
Just the same, it may be ancillary effects that you are noticing - I would guess most of the hunting you do is fairly early in the morning. If that's right, it is worth considering that pressures on the high end are often associated with light winds, fair skies and morning temperature inversions that inhibit vertical as well as horizontal mixing and dilution of gasses and particulates being emitted by a source of interest. So the concentration of an odor being sought/tracked by your dogs might remain higher over a given period of time than it would when pressures are lower and there may be a storm system nearby with clouds, some wind and and a more mixed atmosphere boundary layer.
As for the last part of your question, barometric pressure is the force per unit area exerted in all directions by the air at any given point. For example, the average sea level pressure amounts to about 14.7 pounds per square inch (equivalently expressed as 29.92 inches Hg or 1013.25 millibars). It is proportional to the "weight" of all the air in a vertical column above that location up to the top of the atmosphere. The amount of air in the column depends on the elevation of the observer (lower for someone in a plane 5000 feet above your head than at the ground, for example), the balance of air flowing into and out of that column through the depth of the atmosphere, and the temperature and humidity of the air through the depth of that column. It is measured with a barometer.
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