It would be easy to test changes in various wood characteristics such as hardness, bending flexibility, vibrational sound conductivity, damping, etc. in a barometric pressure test chamber, but it seems unlikely to me that barometric pressure changes due to weather would significantly alter those characteristics. However, barometric pressure changes definitely have short-term effects on the hearing of normal people and could have longer-term effects on the hearing of those with certain abnormalities.

An abstract of a study on the effect of atmospheric pressure on hearing in normal subjects is available here http://www.ncbi.nlm.nih.gov/pubmed/8128881 at the National Institutes of Health US National Library of Medicine. This is taken from that abstract:

"When the pressure was changed to +/- 500 mmH2O at 33 mmH2O/s the results were as follows: When subjects did not equilibrate middle ear pressure, air conduction at low frequency tones increased more than bone conduction. The degree of deterioration in hearing was greater when the chamber pressure was increased (descent) than where pressure was decreased (ascent). When the subjects equilibrated middle ear pressure, little change in the levels of air or bone conduction was observed."

-Bob

Another guitar change I have noticed with greatly increased humidity is tuning-peg functioning. Wooden pegs and the head wood surrounding holes they are seated in obviously will swell if the wood absorbs moisture, and that clearly is what happened, because all the pegs on my two peg-head guitars have repeatedly become very tight. That hasn't been much of a problem, because I have been able to work the tapered pegs further out to relieve excessive pressure each day as I have tuned the guitars. However, if a peg-head guitar was moved from a very dry to a very damp environment and the guitar was not tuned for several days, the pegs probably would break before they would turn.

Now the wood has absorbed more moisture and the pegs have been worked out daily to relieve the pressure, the peg friction is better than it was in the dry Utah environment. It is easy here to set the pegs so they turn easily, but don't slip, but if they were slightly loose in Utah, they would slip, and if they were slightly tight they would bind.

-Bob

Not only do different woods absorb differing total amounts of moisture in the same environments over time, they absorb moisture at different rates of speed. Some woods absorb slowly, but given enough time, eventually absorb as much as other woods that absorb more quickly. The same is true of evaporation rates when damp wood is moved to a dry environment.

A compounding factor is that wood finishes alter moisture absorption and evaporation time-factors. Wood totally covered in water-proof finish would absorb no moisture whereas wood finished in finish that is totally transparent to water would absorb as if the finish wasn't there. Practical guitar finishes are in between those extremes, being neither totally water-proof nor totally transparent to water they alter wood absorption and evaporation time-factors much like thermal insulation alters heating and cooling rates, but not the end temperatures eventually reached given enough time. (Insulation in the attic of a house reduces internal house temperatures by slowing the rate of heat transmission into the house during the day when the sun is shining, so the inside temperature doesn't rise to the outside temperature by the time the sun sets. If the sun didn't set, the inside temperature would continue to rise until inside and outside temperatures were equal.)

The moisture absorption time-factors of some guitar woods obviously are very long, because the necks of my guitars continued to shrink for years after they were imported to the dry Utah environment. There were no noticeable changes in neck widths over periods of a few days as ambient relatively humidities changed due to weather changes, but over periods of months and years the necks all shrunk enough to make the ends of frets protrude and feel uncomfortably sharp as the guitars were played.

Moisture may be absorbed and evaporated more quickly from sound box woods, not only because different types of woods generally are used, but also because the wood is thinner. The time required for wood to absorb all the moisture it ever will absorb depends not only on the type of wood, but on its thickness. Just as a thicker layer of insulation in an attic will increase the insulation's time-factor, outer layers of wood provide moisture insulation to inner layers and the thicker an outer layer is, the longer the wood's total moisture absorption time-factor will be.

Temperature also affects wood moisture absorption time-factors. Warm wood molecules vibrate more rapidly on average and move farther on average than cold wood molecules (molecule vibration due to heat is known as Brownian motion). Increased wood molecule motion helps moisture molecules move into or out of wood more quickly.

Because of these and other factors, guitars that are moved to different moisture environments undergo a complexity of time-dependent changes. Different materials used to make the various parts of a guitar, including not only the sound box and neck, but also braces, the bridge, trim pieces, glues and finishes all have different individual moisture absorption and evaporation rates that depend in part both on component part dimensions and temperatures. Our guitars are "alive" in the sense that they are constantly changing. Even if they are in stable humidity and temperature environments, component part moisture levels are changing due to absorption and evaporation time-factors. My sudden move from a very dry to a very wet environment makes the changes more apparent, but all wooden musical instruments change in a complexity of ways over time even if they are kept in stable environments.

-Bob