Air pressure is caused by all the weight of the air stacking-up on the earth due to gravity, just like the weight of anything else. Naturally, the lower elevations of the earth, then have more air molecules stacked. Denver, Colorado has 5,280 feet less air stacked upon it; and a 16,000 foot elevation mountaintop has 16,000 feet less air stacked upon it. The weight of the air above, pushes down and out upon the air below creating a pressurization that surrounds the whole earth. Air molecules repel each other vigorously, so that they stay evenly spaced and they create substantial power when pushed together.
AIR PRESSURE IS POWERFUL... Air pressure equalizing to become evenly spaced is a powerful phenomenon that creates wind, hurricanes, tornadoes and allows jumbo jets to move forward. We don't often think of it this way, but as a jet begins its path down the runway, the powerful engines pushing air backward causes a gap in the air in front of the jet and with an equal force of air pressure, pushes the jet forward into that space as the air equalizes. At sea level air pressure is 14.7 pounds per square inch pushing downward and sideways. At two square inches it becomes 29.14 pounds. On a 12" ceramic tile on your floor, there is 144 square inches, so there is 2,116 pounds of air stacked on this tile. Why don't we feel it? Because, it is stacked up on our bodies, as well, and we've grown into it. It permeates our bodies. Our bodies are built to overcome the air pressure and a certain amount of gravity, as well, so we jump and run within it naturally.
If we compare 2,116 pounds of air pressure on a square foot of tile at sea level to the amount of air pressure on the same square foot of tile at Denver, Colorado, it is 1,769 pounds of air pressure (12.29 psi X 144 square inches) for a difference of 347 pounds per square foot.
So, if one were to trap sea level air into a container, such as an airplane fusilage and take it to Denver, Colorado, then the amount of pressure on the 21 square foot airplane door (3' X 7') would be 347 X 21 or 7,288 pounds. This is why, if an airplane window would be blown out, a person would go out the window with it.
AIR PRESSURE IS POWERFUL...
Because baseball is played not only at different air pressures, but different air temperatures, the air temperature creates some additional phenomena. Since cold air allows the air molecules to be spaced closer together, then if one were to take the same airplane fusilage and fill it with 40 degree fahrenheit air at sea level and contain it all the way to Denver, Colorado; then, the following would be the pressure numbers inside the fusilage while sitting in Denver with an outside air temperature of 90 degrees:
The added pressure, to the above altitude example, due to the cold air inside the container versus the hot air outside the container (Dr. Douglas Hittle, Professor Emeritus, Colorado State University provided these calculations for this purpose) would be 1.78387 psi (let's round that down to 1.78 additional psi). So, an additional 1.78 psi X 144 square inches = 256 pounds per square foot X 21 square feet = an additional 5,382 pounds on the same door in the above example. If we add those two numbers together: 7,288 pounds on the door from altitude air pressure differences and another 5,382 pounds from temperature differences, we arrive at a total air pressure from altitude and temperature of 12,670 pounds of air pressure on that door. Enough to blow more than one person out of an airplane fusilage under these conditions.
AIR PRESSURE IS POWERFUL... which is why your tires can hold your 2,000+ pound car up off the rims....
Humidity is the great unknown in the world today.........there are as many "old wives tales" that are untrue about humidity as there are "Big Foot" sightings. Humidity is not heavy air. It is actually light air. It feels heavy on our bodies, because our living and breathing bodies feel it. However, humid air has more hydrogen in it and dry air has more nitrogen. Therefore, since hydrogen is lighter than nitrogen, then humid air is light, which is why clouds can rise even when the air within the cloud is cold. Keep in mind, we are not talking about moisture, we are talking about humidity, which is still in gaseous form.
An object such as a baseball can then displace the lighter air molecules more easily, so a baseball or golf ball can fly further through humid air than through dry air. If we examine the weight of humid air and its effect on air pressure, then the mathematics look like this:
If we compare 20% humidity in the air to 80% humidity in the air, (Dr. Douglas Hittle, Professor Emeritus, Colorado State University provided these calculations for this purpose) the difference in pressure is 0.21704 psi with dry air being heavier than humid air. If we insert this into our example above, then 0.21704 rounded to 0.21 pounds per square inch X 144 square inches = 30.24 pounds per square foot (rounded to 30 pounds per square foot). We then use our example of containing drier 20% humidity air at 40 degrees fahrenheit at sea level within our fusilage container and transport it to Denver, Colorado, which in our example shall be 5,280 feet at 90 degrees and 80% humidity and calculate the difference in air pressure inside the container versus outside. We have 30 pounds per square foot X 21 square feet = 630 additional pounds on our fusilage door.
We can now add the pressure differential of 630 pounds from dry air to the 12,670 pounds to see what the total pressure on that fusilage door is. So, we now have 13,300 pounds of air pressure being applied to that door.
AIR PRESSURE IS POWERFUL... which is why firefighters can lift a massive truck off a person with an inflatable air pillow.....and why a 6 ounce baseball can move very differently in different geographical climates.