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Thermoclines

Written by Nevin
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Published on November 13, 2009

At 20’ on our first PADI Advanced Open Water deep dive, I witnessed something remarkable as we descended. Visibility was poor, but I could still distinctly make out a clear demarcation in the water which looked exactly like crinkled glass that was flowing. As I dived through the invisible barrier I was struck by two amazing sensations, firstly the water temperature had just dropped by several degrees, and I felt the chill I entered headfirst into this strange phenomena. Secondly, it was as if I had just put on my glasses, everything suddenly became so much clearer, the murky water was suddenly crystal clear..” –   Students Dive Log entry

The phenomenon that the above AOW Dive Student was describing is known as a thermocline, something that all divers will know about,  as there is no way you wouldn’t notice a thermocline if you have ever dived through one.

A thermocline refers to a boundary of water which separates regions of warmer water from the colder water below. A Thermocline is formed by the effect of the sun, which heats the surface of the water and keeps the upper parts of the ocean or water in a lake, warm. Water near the bottom remains colder as sunlight doesn’t penetrate enough. Cold water by nature is denser that warm water and gravity keeps it under the less dense warm water. This causes a distinct line or boundary between the warmer water which is less dense and the colder denser water forming what is known as a thermocline. The difference in temperature between the warmer and colder waters may be of several degrees and drops even further each meter below you dive.

Thermoclines in lakes get narrower and narrower with the region of cold water growing towards winter, until a point where the cold water is so much that the water turns over mixing both into a constant temperature. However in the ocean,  a thermocline remains  constantly but the depth at which a diver will experience a thermocline will vary on the region, warmth of the sun and the season of the year.

The shimmering crinkled glass effect that a thermocline has is the exact same effect you see when you look at hot air rising of a tarmac road in the summer, caused by the mixing of air/water at two different temperatures.

How does a Thermocline affect diving?

A thermocline affects a diver is several ways, the most obvious being the need to wear a thicker wetsuit if you plan on diving below the thermocline. As there could be a drop in temperature by several degrees, you feel colder faster and loose body heat more rapidly.  If you are wearing a thinner wetsuit it would be advised to remain under the thermocline for a shorter time span, or stay above it.

The second noticeable effect of a thermocline is a sudden increase is water clarity below a thermocline. This often takes new divers by surprise. Caused by the lens effect of the denser colder water, and the fact that algae and plankton, debris and silt, remain in the warmer upper water, leaving the colder denser water most of the time clearer.

The thermocline also affects the kind of marine life a Scuba diver would encounter, as the lesser, algae and plankton found in the colder water results in different kinds of fish you would find. As water temperature has an important influence upon Fish which are all cold-blooded and their bodies are always the temperature of the surrounding water. Most fish would prefer to be above the thermocline and so do divers.