Have you ever jumped into a heated pool to find the water feels cold? Or perhaps the water and air temperature are the same, but the water feels freezing when you get in?
I am fascinated with this relationship between air temperature and swimming because it affects how cold we feel as we swim, especially when entering or exiting the water.
In general, there is no optimal air temperature for swimming. However, water always feels significantly colder than the air temperature, as water is a better conductor of heat and rapidly reduces our skin temperature.
On average, an air temperature that is 2°C (35.6°F) warmer than pool water is most comfortable for swimmers in heated indoor pools, but what about outdoor swimming or unheated pools?
In this article I will cover how air temperature affects water temperature, covering:
- What is the best air temperature for swimming?
- How does air temperature affect water temperature?
- What temperature outside is too cold to swim?
What Is The Best Air Temperature For Swimming?
Air Temperature For Indoor Swimming
For indoor heated pools, maintaining a set air temperature is very important for the comfort of swimmers and to protect buildings from mould and water structural damage.
As a general rule, an air temperature that is 2°C (35.6°F) warmer than pool water is recommended. [source]
There are two main reasons this temperature differential is recommended, and it is essential for pool operators to know:
- It reduces evaporation of the pool water, which will lower the chance of humidity, which can lead to building structural damage through mould, rot and water damage.
- When moving from cold water to warmer air, this is more comfortable for swimmers.
Air Temperature For Outdoor Swimming
For outdoor swimming, be it in an outdoor pool or in lakes and rivers, there is no recommended optimal air temperature for swimming, as we cannot control the air temperature outdoors.
However, it is fundamental to understand the temperature differences between air and water temperatures, so you know what to expect when you hit the water.
Water typically feels colder than the air temperature, unless the water is significantly warmer than the air temperature.
Let’s assume that you are standing on the bank of a river, with an air temperature of 15°C (60°F). This may feel pleasant.
You know that the water temperature of the river is also 15°C (60°F), so you jump in.
When you hit the water, the water will feel shockingly cold. In fact, 15°C (60°F) is very cold water and you will gasp as you experience cold water shock.
Why Do Air Temperature And Water Temperature Feel So Different?
So why do an air temperature and water temperature that is exactly the same feel so different?
When we stand on the river bank at an air temperature of 15°C (60°F), our skin temperature will be much warmer than this. Air is not a great conductor of heat, so heat does not rapidly leave our bodies as we stand in our swimsuits.
The air in contact with our skin will cool us, but it takes some time to get cold. It is not instant.
Water is a far better conductor of heat compared to air. When we jump into the river, the 15°C (60°F) water will rapidly cool our skin.
Our skin will feel cold instantly, and will not heat easily as more cold water washes over us, which keeps removing any surface heat our skin may have.
Because of the principles of heat transfer, the heat moves from the hotter body to the colder body, so our warm bodies will lose heat in the cold water. Because water is dense, we lose this heat very quickly, so the feeling of cold is intense and happens fast.
This is why neoprene wetsuits are so effective at keeping us warm in cold water, as the neoprene traps a layer of water between our skin and the neoprene.
Our body temperature can heat this layer of water, and as it is trapped, it will keep us warm, acting as a barrier from the cold water.
How Does Air Temperature Affect Water Temperature?
For large bodies of water, such as swimming pools and lakes, there is a complicated relationship between air and water temperature.
The deeper subject of thermodynamics is far beyond the scope of this article, so I will stick to the basics.
Generally, air temperature and water temperature will neutralise to be the same. For example, if you leave a hot cup of coffee on the kitchen counter, it will cool down to become the same temperature as the surrounding air.
With deeper and larger volumes of water, the air will only affect the surface water.
For example, if you swim in an unheated pool on a hot summer’s day, you find that the first few inches of water depth are warm, but as you move deeper, the water will be much colder as this water is not in contact with the sun rays or warm air.
As the first few inches of water may be warmer than your skin, you perceive this as warm, but as you move deeper, heat loss will occur in the colder water as the cold water takes heat from your body, which you feel is cold.
At the end of the day, it is all about heat transfer. As water is better at conducting heat than air, we often perceive it as being much warmer or colder than air.
What Temperature Outside Is Too Cold To Swim?
As mentioned, water can feel much colder than the air temperature, even if they are similar temperatures. Therefore, you cannot rely on air temperature alone for a safe swim. You must know how cold the water is.
Any water temperature that is below 21°C (70°F) should be treated with caution according to the National Centre For Cold Water Safety. [source]
To learn more about what to expect from cold water, and what different water temperatures feel like, including knowing when it is too cold to swim, I have covered this in far greater detail in this article, Swimming In Cold Water (What to expect as a beginner)
Final Thoughts
As swimmers, we can be so focused on water temperature that we forget how air temperature can affect how we perceive the water and our swimming comfort.
In summary, if the air is the same temperature or slightly warmer than the water, the water will still feel noticeably colder to us.
This is because water is a much better heat conductor than air and will take heat away from our bodies, which we interpret as getting cold.
Despite air temperature, it is fundamental to know the temperature of the water that you plan to swim in, as cold water can be deadly as cold water shock can occur.
As a general guide, treat any water that is less than 21°C (70°F) with caution. To learn more about cold water temperatures and what they feel like, check out this article, Swimming In Cold Water (What to expect as a beginner)
Happy swimming!
