Water pressure is, in fact, greater than air pressure. If air is squirted in, the air pressure must be greater. But, because we know the water will shoot out, we know the pressure must be higher. Of course, this is under typical circumstances. If the water source gets shut off, then there would be no pressure.
Resistance is how much it takes to move something. Water is resistant because it flows until something stops it from flowing further (a river), and air is resistant because it moves around objects (air). If you were to compare the two, water is harder because it's not as easy as just pushing something away from you. It needs a path of some kind to flow through.
The more resistant something is, the harder it is to push it. This is why things like rocks are hard to move by hand but easy to move by machine because machines can do lots of resistances at once while humans can't.
It's also why trees are hard to go under but easy to go over. They're mostly made up of water which is hard to push away from you but easy to run right past them.
And finally, it's why animals' skins are hard enough to keep them safe from harm but thin enough for the wind to blow through.
Consider this: if you made a hole in the side of a plastic container full with water, would water spurt out or air squirt in? Contrarily, if water were squirted in, it would not come out because there is no force behind the flow of water.
Air pressure is equal to the weight of one square inch of air pressing on every square inch of surface area at that height. For example, if you weigh 150 pounds and stand on a foot pedal, what is the pressure inside your shoe? This depends on how high up you are standing; if you were standing over 10 feet tall, the pressure would be very high.
The pressure of water is called hydraulic pressure and it's measured in pounds per square inch (psi). The pressure of air is called atmospheric pressure and it varies with altitude but is usually given in inches of mercury (inHg). At sea level, it is 14.7 inches of mercury (inHg) for every square inch of surface area.
As you can see, water is well-equipped to deal with problems that air cannot. If you were to cut yourself on a knife blade, the blood would quickly drain away from your body because it is carried by water first to the lungs where it gets filtered out of the bloodstream.
Water is a liquid, whereas air is a gas. For all intents and purposes, air is compressible, but water is not. Water is H2O with some dissolved minerals and gases, whereas air is a gas combination with some water vapor. Air is more dense than most liquids or gases, so it floats on top of them.
There are many other differences between air and water that we will discuss below. But first, let's take a look at how they compare in terms of density.
Density is the measure of the mass of a substance per unit volume. Denser substances occupy more space than less-dense ones. Air has a very low density because there are so many molecules in such a small space (compared to water molecules). Just like any other molecule, each air molecule is made up of two parts: an oxygen atom and a hydrogen atom. These atoms are very light and can move around relatively freely inside the gas mixture. When you add these atoms together in large numbers, they fill up your container or bubble bath and don't leave much room for anything else.
The next table shows you the relative densities of some common substances. You'll notice that although air is relatively low-density, it still weighs more than water. This is because there are also lots of empty spaces in the atmosphere filled with other gases such as nitrogen which make air heavy compared to water.
In our sector, pressure testing with air vs pressure testing with water is a standard procedure for detecting leaks. We are frequently questioned why air testing requires lower pressures than water testing. A low-pressure (30-50 PSI) air test is just as useful as a high-pressure (150 PSI) water test for detecting leaks. In fact, an air test can detect smaller holes and cracks than a water test.
The reason air tests require lower pressures than water tests is because air is less dense than water. This means that there will be leakage even if the hole or crack is not visible. The only way to find these hidden leaks is by using lower pressures during the test.
Also, water leaking into buildings causes damage all by itself. It can cause flooring and ceiling materials to rot. It can also lead to structural damage if the load-bearing capabilities of certain parts of the building are being compromised. Water that escapes through plumbing connections is called "potable" water because it is safe to drink. Non-potable water that escapes through plumbing connections is called "waste" water because it isn't safe to drink or use in any other way. Potable and non-potable water have different requirements when it comes to testing for leaks. Only leaky pipes containing potable water should be tested for pressure; otherwise, you could end up causing more damage than fixing by over-pressurizing them!
Finally, air tests are easier to do.
Air pressure is actually greater than water pressure at sea level. That is, the pressure exerted by the water must exactly match the pressure exerted by the air. At higher elevations or in warmer climates, the pressure of the air is less than that of the water, so the fish have more time to breathe before they drown.
This is because water molecules are heavier than air molecules. So when there is more space between them (like at high elevations or in warm climates), the pressure of the water is less than that of the air. Fish need oxygen too! So they use their gills to take in breathable air just like humans do with our lungs.
Fish swim faster through air than water because there is less resistance from the water. But still, even fish can't breathe for very long without water flowing over their gills.
And finally, water is required for life as we know it. We need water to live and fish need water to survive. So they cannot live without each other.