What Are The Differences Between A Halocline And A Thermocline?

We tell you about the differences between a halocline and a thermocline. Many divers have been puzzled when for the first time during a dive they have crossed a layer of water and suddenly see a blur, they blink to clarify their vision and open their eyes again to have the same blurred vision. As we continue to descend, as suddenly as the blurred vision appeared, it disappears, and we again see perfectly. This first experience can be puzzling if we do not know why it is and some divers have thought that they could be experiencing symptoms of narcosis.

It actually is a physical phenomenon called thermocline or halocline consisting of two layers of water with different qualities of temperature or salinity. This means that the layers do not mix due to their different physical properties and when we go through the separation layer, we mix the two types of water, producing light refraction phenomena that give us such a perplexing sensation of blurred vision.

Differences Between A Halocline And A Thermocline; Why Does A Thermocline Happen?

Why are these layers produced? Each type occurs for a different cause. The thermocline is a layer of separation between two bodies of water at different temperatures. The sun heats the surface of the sea or lake, hot water is less dense than cold water, so it floats on top of cold water.

It is somewhat similar to hot air balloons that by heating the air inside them it is less dense and the balloon rises. Also, hotter water on the surface gets hotter by being in direct contact with sunlight and gets hotter by increasing the temperature difference. The greater the temperature difference between the two layers, the stronger the effect.

In tropical areas where there is very strong insolation all year round, this layer is not usually noticeable at recreational diving depths. In the Caribbean, it begins to be noticed from 80 meters/262 feet deep, and we only get there with specialized technical diving techniques. In seas, like the Mediterranean, it goes down throughout the summer and stabilizes around 20 or 25 meters/  66 or 82 feet towards the end of the summer.

Differences Between A Halocline And A Thermocline; Why Does A Halocline Happen?

A halocline is also a layer of separation between two water masses by difference in density, but this time it is not caused by temperature. It occurs when two bodies of water come together, one with freshwater and the other with saltwater. Saltier water is denser and sinks leaving fresh water on the surface. This effect is difficult to observe and usually occurs in areas where freshwater reaches the sea, such as caves with freshwater upwellings that discharge into the sea.

The best and most spectacular place to observe this effect is in the Cenotes of Mexico. The Riviera Maya area (Yucatan peninsula) is formed by rock from ancient fossilized corals that is very porous as if it were a sponge, this results in all the rainwater being absorbed by the rock and run through to the underground rivers that lead towards the sea.

Seawater also seeps into the rock and at one point meets with the freshwater forming a separation layer, consisting of a halocline and also a thermocline, since in addition to the difference in density of seawater and freshwater, the seawater is also hotter than the freshwater which adds up the two effects amplifying the result. This layer is found in most Cenotes near the coast at around 10 meters/33 feet deep.

If we combine this strong layer with a visibility of more than 60 meters/ 197 feet in an environment full of stalactite and stalagmite formations of different colors, we will understand why the Cenotes are a world pilgrimage site for divers who even give up diving on the magnificent Mexican reefs to spend a whole week diving in different cenotes.

Now you know the differences between haloclines and thermoclines, there is no need to take special precautions when going through them, just enjoy the effect. If you are dizzy and uncomfortable, simply, simply descend or ascend a little since the area of influence of the effect is of just a meter/3 feet. I personally love to dive in the Cenotes just below the layer and swim very slowly observing how an oil-like wave mixed with water is produced that spreads out half a meter above me.

We already know the differences between haloclines and thermoclines and that they are great natural phenomena of intense beauty. Really, we are in luck divers, because we can enjoy both of them and their visual effects in different environments.

Article written by Víctor Cordoba: Course Director and Dressel Divers’ Head of Human Resources.