At sea level atmospheric pressure is:

1 atm = 1.013 x 10⁵ Pa

This is an average value, and the pressure fluctuates somewhat. In good weather the pressure is higher than average and in bad weather it is lower than average.

Its low density notwithstanding, air does apply a force of over 100,000 N on each square meter of area. We can see the effect of this force by removing the air from inside a hollow object - then there's no outward force from air on the inside to balance the inward force from the air on the outside. This can have a dramatic, and rather damaging, effect on an object.

A static fluid is one that is at rest. Two very important things to remember about pressure in a static fluid are:

1. Pressure increases with depth.
2. Two points at the same vertical position experience the same pressure, no matter what the shape of the container.

If point 2 is a vertical distance h below point 1, and the pressure at point 1 is P₁, the pressure at point 2 is:

P₂ = P₁ + ρgh

Point 2 does not have to be directly below point 1 - all that matters is the vertical distance. Any horizontal displacement is irrelevant.

Click and drag the sensors to measure pressure. The red sensor measures correctly only in air; the purple one measures correctly only in the water.

The relationship between pressure and depth is exploited in manometers (or barometers) that measure pressure. Pressures are measured by comparing the pressure at one end of a container with a known pressure at the other end.

A standard mercury barometer to measure atmospheric pressure is a tube with one end sealed. The sealed end is close to zero pressure, while the other end is open to the atmosphere. The pressure difference between the two ends of the tube can maintain a column of fluid in the tube, with the height of the column being proportional to the pressure difference. If the closed end has negligible pressure, the height of the column is proportional to atmospheric pressure being measured.

Mercury is commonly used in barometers because its high density means the height of the column can be a reasonable size to measure atmospheric pressure. A barometer using water, for instance, would need to be 13.6 times taller than a mercury barometer to obtain the same pressure difference. This is because mercury is 13.6 times more dense than water.

In an open-tube manometer, one end of the tube is open to the atmosphere, and is thus at atmospheric pressure. The other end is connected to a region where the pressure is to be measured. Again, if there is a difference in pressure between the two ends of the tube, a column of fluid can be supported in the tube, with the height of the column being proportional to the pressure difference.

The actual pressure is known as the absolute pressure; the pressure difference between the absolute pressure and atmospheric pressure is called the gauge pressure. Many pressure gauges give only the gauge pressure.