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  • What is Thermocline?

    Last Updated: January 5, 2021

    If you’re new to scuba diving, you’re about to be introduced to a range of different terms and phrases that you might not have heard before.

    The word “thermocline” is probably one of them, but don’t worry if you have no idea what this means - we’re about to explain it to you.

    After reading this useful guide on thermoclines, you’ll know exactly what people are talking about the next time you stumble across the term thermocline, or how to tell if you swim right into one. 

    So, let’s dive right into it. 

    What is a Thermocline? 


    A thermocline is the section of water that falls between the top surface layer (epipelagic zone) which is warmed by the sun, and the lower depths of the sea which are much colder in temperature. Think of it as the jam to the ocean’s Victoria Sponge cake.

    Defined by Merriam Webster as a “thermally stratified body of water”, or simply referred to as “clines” by experienced scuba divers, a thermocline is essentially the layer of water where different temperatures meet in the middle. 

    Because of the difference in the bordering layers, the water inside a thermocline rapidly cools and causes an abrupt change in temperature that you can feel when you pass through it.

    Not just exclusive to oceans, thermoclines can be found in a number of different bodies of water, including lakes and oceans, and sometimes at the crosspoint between two. 

    Where Does the term ‘Thermoclines’ Come From? 

    From an etymological standpoint, the term thermocline stems from two words: ‘thermal’ and ‘cline’. While most people are probably familiar with thermal and its meaning (heat), the less obvious ‘cline’ means layer.

    I guess it’s pretty obvious how it got its name when you consider the definition of a thermocline. 

    When it comes to the ocean, the word ‘clines’ can be used generally to describe sections of water with properties that are drastically different from the adjacent layers. As we’ve already explained, thermocline exclusively references the difference in temperature. 

    Also belonging to this group are haloclines and pycnoclines, which refer to salinity or salt content levels and water density respectively.

    The former types of cline are most often observed at the meeting point between rivers and oceans, whereas the latter can occur in any body of water with varying degrees of temperature and salinity. 

    Why Do Thermoclines Vary?

    Thermoclines can be found at various depths and in different locations depending on the type of water they occur in.

    The strength of a thermocline also differs, so you may find some that are milder or practically unnoticeable, whereas some will give you a shock as you swim through them due to the drastic change in temperature they create. 

    Seasonal change hugely affects the formation of thermoclines and how they vary. For example, warmer climates that experience sunny weather all year long are likely to have pretty consistent thermocline depths, and the same goes for colder regions.

    However, if you live somewhere that has changing seasons that fluctuate in terms of weather temperature, there’ll be less regularity between thermoclines.

    During the warmer months of the year, they’ll typically form further below the surface due to the heating power of the sun being able to reach lower depths of the ocean.

    When it gets colder, the same area of water can cause thermoclines that are much shallower, if they form at all.

    How Do You Spot a Thermocline?

    Usually, people say that if you experience a sudden drop in temperature that sends shivers down your spine, it means someone’s just walked over your grave.

    If this happens when you’re swimming or diving, don’t panic - you’ve probably entered a thermocline.

    There are other ways you can recognize a thermocline, but passing through a cold patch of water is the most obvious sign.

    As well as physically noticing a thermocline, you can look for visual clues in the water. 

    Thermoclines are known for the distinctive “shimmer” they create. Think of the subtle distortion of the air that’s visible just above the surface of the road when you’re driving on a hot day because a thermocline is similarly elusive. Blink, and you might miss it! 

    The noticeable change in the water where a thermocline exists is caused by the light refracting (if you skipped one too many science classes as a kid, this means changing direction) which causes the perceptible manipulation of the sea’s surface. 

    Can You Dive in a Thermocline? 

    Of course! Thermoclines don’t interfere with diving in any way, and you’re not guaranteed to run into them (or swim into them, in this case) every time you’re in the water, but it can still be useful to think about whether or not you’ll encounter them on your dive before you set off. 

    This is so you can plan for unexpected cooler waters and wear your most appropriate wetsuit.

    Depending on how long you’re planning on being down there for, keeping warm doesn’t only make you more comfortable during a dive, but it also helps to conserve your air.

    One of the first things you’re taught when you’re trying to get your open water diving certification is what can cause you to consume your precious air supply faster than normal, and this includes being cold as your body has to work harder to keep you warm. 

    So if you’re expecting to experience a thermocline, you might want to put on your best wetsuit! Otherwise, your dive may be cut short, and as anyone who’s been diving before will tell you, every second under the sea counts. 


    If you’re new to scuba diving, you’re about to be introduced to… wait, does this sound familiar? In all seriousness though, alongside all the new terminology you’ll learn, you’re about to experience to a magical underwater world that’s full of amazing natural wonders. 

    We might have only just scratched the surface of what exists in our oceans, but thermoclines are one thing we can understand, and now, after reading this article, so do you! 


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