What would be the effects of a change in air pressure in the atmosphere?

Lower pressure would make it difficult to breath, like being at very high altitude. You would have to breath more rapidly to get enough oxygen.

Increased air pressure up to maybe 2 bar (Earth is currently 1 Bar, about 14psi I think, at sea level) might be a good thing because you should get more oxygen in each breath.

But increasing much more than that would probably become a bad thing again at some point. Even though your body would equalize with internal and external pressure, just like when you deep dive with tanks, at some point I'm sure the body would be overcome by too much pressure.

The air pressure is constantly changing and we get wet weather when the pressure is low and clear skies when it is high

But if you mean a large change I can answer you this way. Many years ago I worked in a high altitude laboratory and we had chambers to simulate both height and depth. It was thought at the time that Multiple Sclerosis could be improved by placing victims in high air pressure to force the oxygen deeper into their veins and muscles. I was the lab first aider so had to go in the chambers with the patients. The man outsideslowly increased the pressure to over two bar, that is twice standard atmosphere. It did me no harm but I was hyper active for an hour afterwards due to the enrichment by oxygen - a great feeling

Anyway since you are in the global warming section maybe I should tell you its a hoax, just in passing :-)

There's a couple of ways of looking at this...actually a bunch. The most basic would be a change in either volume, temperature or both: http://www.grc.nasa.gov/WWW/K-12/airplane/eqstat.h...

Which can be rewritten to represent density,mass, joules, atomic weights or emphasizing a constant. There are questions concerning molecular attractions the interactions between certain compounds/substances at vary pressure ranges and distances. Perhaps the best example would be the temperature change at certain altitudes and the boiling point of water (Dave H) gave one example.

Another way would be to explore the complexity of highs-lows/negative-positive pressure on weather systems: http://en.wikipedia.org/wiki/Low-pressure_area

Trying to express any type of singularity, may prove a difficult task.

Maxx is pretty much right, but he failed to state that with a higher pressure on the body, over long periods of time, you internal organs would change. Like the divers that don't use scuba gear, the lungs become more efficient but shrink in size.

comparable reason that submarines are subjected to greater and better exterior pressures attempting to crush the deliver. the rationalization is, the deeper you flow, you're "feeling" the rigidity of that far greater water above you as you dive deeper. to place it in greater comprehensible words, there is a lot greater "weight" of water above you as you dive deeper and you're able to sense that "weight". comparable difficulty is going for atmospheric rigidity. status on the floor (as sea point, say) you would be feeling the rigidity of each and every of the ambience above you. As you ascend into the ambience and there is far less ecosystem above you, there is far less rigidity, or "weight" in case you will, pushing in at you.

If you only change pressure, and no other variables (e.g. temperature and relative humidity) you will get this.

Lowering the pressure lowers the 'dew point' temperature of water vapour. The 'dew point' is the temperature at which water vapour will condense out to form water droplets and become clouds.

Lower pressure will cause more cloud to form and allow clouds to form at lower altitudes, even down to the ground (fog). Higher pressure will reduce the amount of cloud and cause them to form at higher altitudes.

global warming

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