Atmospheric pressure

What is the significance of atmospheric pressure being referenced as 100 kPa ? Does that really mean 10000 kg/m$^2$ acting upon us ?

If we are measuring only collision force on the surface and not the weight of the entire column of air above us, then why figure 100 kPa (100000 N force/m$^2$) which is huge and significantly more than what is required to raise mercury level by just 76 cm.

2 Answers

Let's work out the force exerted per square metre of $0.76m$ of mercury. The density of mercury at room temperature is 13591 kg/m$^3$, so the mass of a block of mercury 1 square metre by $0.76$ metres high is $10329 kg$, and multiplying by the acceleration due to gravity $9.81 m/s^2$ gives the weight as $101325N$. The pressure is therefore $101325Pa$, which is one atmosphere.

So one atmosphere is exactly the pressure exerted by $0.76m$ of mercury.

Keep in mind that pressure is not force. Also, pressure is not mass per area. Pressure expresses the ratio of differential force exerted normal to a surface per differential area, and is expressed in units of force per area.

A pressure of 100 kPa exerted on a surface means that on 1 cm$^2$ of that surface there is a total force of 10 N normal to the surface of that square centimeter. One must remember, however, that there may be a pressure applied to the other side of that surface resulting in a force in the opposite direction.

Pressure is not a vector, and in our gravitational field, the local pressure at a fixed altitude is roughly constant. The force on a surface due to atmospheric pressure is always normal to the differential surface area, so sometimes the resulting force is up, sometimes down, sometimes sideways.

Differences in and changes in temperature of the atmosphere introduce changes and differences in air density which create horizontal pressure differences. This is what causes wind, resulting in weather systems moving across the globe. But the mean atmospheric pressure remains very close to 101 kPa (1013 mbar) near sea level. Hurricanes/cyclones have central pressures in the 90-98 kPa (900-980 mbar) range, with lower pressure being stronger storms.