Question

Why does atmospheric pressure usually decrease when humidity increases?

Answer 1

Atmospheric pressure usually decreases when humidity increases because moisture in the air displaces heavier nitrogen and oxygen molecules, reducing the weight of the air in the area.

Explanation:

The change due to humidity may be measurable, but is usually slight. Humidity generally increases as a result of warmer air moving into the region which can bring in water vapor with it, or generate it through evaporation.

So the effect of the air becoming increasingly humid is less of a factor than the temperature of the air carrying the water vapor.
Warmer air will always result in reducing the atmospheric air pressure when replacing cooler air.