If you watch MySwimPro YouTube videos, you’ve probably heard Coach Fares say that water is 800x more dense than air. But did you know that water density can vary, depending on a number of factors such as elevation, temperature and more?
Let’s dive into the science of water density, and how you can swim more efficiently, reduce drag and go faster with the right technique.
Why Water Density Matters to Swimmers
Swimming is tough! Because water is super dense, swimmers need to refine their technique to maximize efficiency and speed. If you want to swim faster, you need to learn to work with the water, not against it. You can do this in two ways:
Related: How to Decrease Drag in Swimming
- Decrease Drag: Work on proper technique and body position so that your body displaces as little water as possible.
- Increase Propulsion: Get stronger so you pull and kick more water with every stroke.
Finding the balance between drag reduction and propulsion is key to swimming faster without getting as tired. When you understand how your stroke is interacting with the water, you can make improvements that will completely change the way you move through the water.
Now, let’s get into the numbers!
What is Density?
Density is the ratio of the mass to the volume of a substance. The formula for density is:
ρ = m/V
- ρ = Density, units are typically kg/m³ or lb/ft³
- m = Mass, units are typically kilograms or pounds
- V = Volume, units are typically m³ or ft³
Comparing Air & Water Density
Water’s approximate density is 1 gram/ml. If we’re getting specific, water’s actual density is 0.9998395 g/ml at 4.0° Celsius (39.2° Fahrenheit).
The density of dry air is 1.225 kg/m³ or 0.001225 gram/ml at sea level and 15° Celsius (59° Fahrenheit). Dry air is made up of 78% nitrogen and 21% oxygen with the remaining 1% being made up of various gasses, including argon, carbon dioxide, neon and helium.
Related: 4 Ways to Swim Faster With Less Effort
Air density can vary depending on the amount of water vapor in the air. The more water vapor there is in the air, the less dense the air is.
So, knowing that air’s density is 0.001225 gram/ml and water’s density is 0.9998395 g/ml, we can accurately calculate that:
Water is 816.1955x more dense than air!
But it’s not always that simple. Water density can change! Let us explain…
What Causes Water Density to Change?
Water density can change due to one of the following factors:
- Temperature: Ice is less dense than liquid water. This is why ice floats!
- Salt: As salt dissolves in water, it adds mass, which makes the water more dense and allows more objects that would normally sink in fresh water to float. This is why it’s so easy to float in the Dead Sea!
- Depth: Water’s density increases with depth. If you’ve ever gone scuba diving or swum down to the bottom of a deep pool, you’ll feel more pressure.
- Elevation: Depending on your altitude, water may be more dense relative to the density of the air where you are.
Comparing Different Densities
Let’s take a look at a few instances where water ends up being even more dense than air!
Related: How Many Swimming Laps are in One Mile?
Baseline Water Density:
- Water @ 79° F (26.1° C) = 0.99676 gram/ml
- Water is 813x more dense than air
Sea Water Density:
- 1020 to 1029 kg/m³
- 1024.5kg/m3 = 1.0245g/ml
- Water is 836x more dense than air
Water Density at the USA Olympic Training Center:
- Altitude: 1,935 meters (6,350 ft)
- Air Temp: 24° C (75° F)
- Water: 0.99676 gram/ml
- Air: 0.0006836
- Water is 1,498.68x more dense than air
Water Density at the Top of Mt. Everest:
- Altitude: 8,848 meters (29,032 ft)
- Air Temp: -30° C (-23° F)
- Water: 0.99676 gram/ml
- Air: 0.0001162
- Water is 8,816.69x more dense than air
What do you think? Have you experienced changes in water density between the pool and the ocean? Share your thoughts in the comments! Check out these calculators for water density and air density.
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1 Comment
Water density does not increase with depth, since water is not compressible. The pressure increase a diver feels is from the increase of absolute atmospheric pressure: in sea water about .445 psig per foot, the value of 1 atmosphere (atm) of 14.7 psia at sea-level.