Joe’s PADI IDC and Divemaster classroom

Joe’s PADI IDC and Divemaster classroom

Heat and Water in theory:

As we learned, water absorbs heat faster then air does. But why is that? There are several reasons why heat absorption is greater in water. One of them is that water is denser or has a greater thickness of matter than air. It also means water has more mass = molecules for a given volume. Heat is the energy what gets molecules moving. Hence the denser (more molecules) a substance is, the more energy (heat) it needs, which means it absorbs heat faster then a less denser substance like air for example. Another point is also the bonds within the molecules which are so called weaker or more elastic which needs more energy (heat) to overcome. So to summarize, water needs more heat energy to rise to a given temperature.

Heat and Water in a practical approach:

The ability to absorb heat has a big impact on divers in water. Through heat conduction and convection, water takes away heat from the body approximately 20 times faster than air does.

Heat transmits from one substance to another in three different ways: conduction, convection and radiation.


Conduction refers to heat transmission via direct contact. The best example here is to put a metal spoon in a hot cup of tea. After a few moments not only the submerged part of the spoon is hot, even the handle becomes hot. A substance that easily transmits heat this way is called a good conductor. So that’s why you need proper insulation under water as water is conducting heat away from your body 20 times faster then air does.


Convection involves heat transmission via fluids. As hot (less dense) water tend to rise it needs to be replaced by cooler (heavier) water. For example, you are diving in cool water without a wet suit, your skin would heat the cool water surrounding you and make it less dense. This water will rise as it is less dense and you will find yourself surrounded by cold water again in a continuous flow.


Radiation refers to heat transmission via electromagnetic waves. Which is the least of our concerns underwater. This kind of heat you normally experience from a fire place or the sun. You might experience this kind of heat from underwater strobes or powerful flashlights.

Summary for diving:

If you own a wet suit which is not fitting properly which provides a bigger layer of water in-between you and the neoprene, more heat will be conducted from your body as the bigger amount of water needs more energy and absorbs it from your body. After the layer of water is heated up and less dense, it now will escape your poor fitting wetsuit and will be replaced by cooler water due to convection.

And this alone with a drawing on the white board most likely explains to your students why they felt cold on the last dive.

Stayed tuned for our next Blog in Joe’s PADI IDC and Divemaster Classroom when we’ll talk about Physiology.

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