hydrostatic pressure

Hydrostatic pressure is the pressure exerted by a fluid at rest due to the force of gravity. It’s the pressure you feel when you dive to the bottom of a swimming pool.

Hydrostatic pressure applies to all fluids, meaning all liquids and gases (a fluid is any substance that can flow and change its shape). Hydrostatic is a word used to refer to a fluid that is not in motion. So, hydrostatic pressure means the pressure “inside” a non-moving fluid—not the pressure exerted by, say, the water coming out of a fire hose.

Hydrostatic pressure affects many crucially important aspects of modern society, including staying safe in space and deep water. Also plumbing.

That moment in every submarine movie when they go too deep and the walls start to cave in? Yep, hydrostatic pressure. In water, for every 33 feet you go down, the pressure increases by 1 bar, or 14.5 pounds per square inch! The lower you go in a fluid, the more intense the hydrostatic pressure gets. That’s because gravity pulls on the substance at a constant rate. Whether you’re in a swimming pool or the depths of the ocean, the hydrostatic pressure is greatest at the bottom because of the weight of all the water above. In other words, the deeper you go, the more water there is above you to press down.

Density also plays a role. The more dense a fluid is, the greater its hydrostatic pressure is. There’s a precise formula for this: P = r * g * h, where P is pressure, r is the density of fluid, g is the acceleration of gravity, and h is the height of the fluid above.

Hydrostatic pressure is why deep-sea diving requires special safety equipment. Our bodies are not specialized for the high hydrostatic pressure of the deep ocean. Instead, we’re optimized to operate at the lower hydrostatic pressure of the air at the surface of Earth—1 bar. The hydrostatic pressure of air also changes depending on elevation, which is why the air is “thinner” at higher elevations (like at the tops of mountains or in an airplane).

Remember: any static fluid (air, water, helium, sulfuric acid, root beer) has hydrostatic pressure, and we see it in action all the time in the world around us. Hydrostatic pressure is why oil floats on water, and why bubbles float to the surface of your root beer: gravity exerts a higher pressure on the denser substance, which pushes the less dense substance to the top. Hydrostatic pressure is also important for our bodies—it’s crucial to processes like blood flow.

Hydrostatic pressure is one reason (along with the lack of oxygen) why it’s not safe for humans to travel unprotected in space. You rely on hydrostatic pressure to keep your lungs at the right inflation and to keep the water in your body from vaporizing.

Hydrostatic pressure affects all kinds of fluids, including those that we spend a lot of time in: air and water. Here are some more details about how that works.

The Lampert's Sea Cucumber moves around using its water vascular system that operates by hydrostatic pressure. pic.twitter.com/r86Y2sYXqb

— GonnaDive.com (@GonnaDive) February 9, 2016

My life follows the hydrostatic pressure law. When it goes down, pressure increases.

— Bryan See (@bryan_see0102) September 29, 2019

• fluid mechanics
• osmosis
• hydrostatics
• hydraulics 
• hydrodynamics

You’re about to scuba-dive. Where will the hydrostatic pressure be at its greatest?

A. 5 feet below the surface of the water.
B. 50 feet below the surface of the water.
C. 500 feet below the surface of the water.
D. 5,000 feet below the surface of the water.