NWS JetStream - Air Pressure
Increasing the volume of a container of air will cause the air pressure inside the container to decrease (rate of collision between air molecules and container. It seems like there is an indirect relationship between air pressure and relative said that the average water content of the atmosphere is about 2 grams per kg. Atmospheric pressure, sometimes also called barometric pressure (after the sensor), is the .. damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding gas or liquid .Wind and Air Pressure
This would seem to explain your discovery that humidity in inversely proportional to air pressure. If I can come up with a better answer I'll let you know. I think a better way to think about relative humidity is that it is the ratio between the amount of water in the air and the amount of water that can be stored in the air.
The amount of water that can be in air depends on temperature.
UCSB Science Line
Colder air can hold less water than warm air. It turns out that in a high pressure area, air is flowing out to areas of lower pressure. This air is replaced by air from the upper atmosphere which is colder and thus drier. So people inside a high pressure area notice that the air is drier than usual. Likewise, low pressure areas draw in warmer, moist air so people in a low pressure area would notice higher humidity.
Storms occur when a cold air mass and a warm air mass meet.
Since the warm air and cold air don't mix very well, one air mass rises and the other doesn't. Which air mass rises? When this air mass rises to high altitude what do you think happens to its temperature? What is the relative difference in air density between these two altitudes?
The air density at 20 km is only a third of that at 12 km. The high speed of supersonic aircraft is made possible by the reduced air resistance at 20 km. Consider a coastline with initially the same atmospheric temperatures and pressures over land L and over sea S.
Assume that there is initially no wind. In summer during the day the land surface is heated to a higher temperature than the sea.
This difference is due in part to the larger heat capacity of the sea, and in part to the consumption of heat by evaporation of water. Oxygen is second, at around 21 percent. Argon, which comes in third, makes up only 1 percent of the Earth's atmosphere. All other gases normally exist in proportions of less than 1 percent -- except for highly variable water vapor.
Shifting Gases The amount of air composed of water vapor is typically 1 to 4 percent.
All gases in the air, including water vapor, exist in constantly shifting proportions. Since their total must equal percent, an increase or decrease in the percentage occupied by water vapor results in a decrease or increase of the percentages of other gases. Since atmospheric pressure at sea level is always approximately 1 bar, an increase in water vapor at any given location changes it very little.