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Diving in Cold Water

The sea around the UK is rarely colder than about 8°C, but that's not true of inland freshwater sites where significantly lower temperatures can be encountered i.e., 4°C. Learn the "stay warm" tricks of the all-year-round divers.

Classifying Cold

There are three primary factors that affect one’s tolerance to cold which are:

  1. Physiological - a person’s body shape, size and circulation
  2. Equipment - how effectively the body is insulated and
  3. Psychological - one’s perceptions and expectations.

All three factors are separate drivers that are assessed and addressed differently and yet all three are inextricably linked.

There is no specific point at which warm water becomes cold water or when cold water becomes extreme cold water, as any such distinctions are dependent primarily upon one’s body, experience, equipment and attitude. Temperatures from 20°C down to 7°C would be the average range that most cold water divers would dive with wetsuits at the top of the spectrum and usually in drysuit's towards the bottom of the range.

Below 7°C down to 4°C is when most find the water very cold, even with drysuit divers may emerge chilled.

Technical divers with extended decompression requirements may spend hours submerged in cold water and as the length of exposure increases, so does the need to take special measures.


Hypothermia is a condition in which the deep tissue or core temperature of the body falls below 35° C, which is the temperature at which malfunctions in normal physiology begin to occur. If the core temperature drops below 36° C, diving operations should be terminated because the consequences of continuing are serious. If the core temperature falls to 34° C, temporary amnesia may occur and emergency rewarming and medical treatment are required. Between 30° and 32° C, cardiac irregularities commence and unconsciousness may result.

Because water has a specific heat approximately 1000 times greater than that of air and a thermal conductivity 24 times greater than that of air, the body looses heat much faster in water than in air of the same temperature. Fortunately, the thermo regulatory system of the body is highly sensitive to stimulation from the hands and feet, so that the body's heat generating systems are activated before the core temperature is affected seriously. The fact that the hands and feet get cold first is thus, in this sense, an advantage.

With cold skin and with core temperatures below 36° C, the defence mechanisms of the body are activated. These mechanisms consist of shivering, which can increase basal body heat production by up to five times, and vasoconstriction, which reduces blood flow to the periphery and thus reduces heat loss. Unfortunately, these mechanisms rarely achieve heat balance, so that the diver continues to loose heat.

In addition to loosing body heat by conductive loss from the skin, a significant loss (10 to 20 percent of total body heat loss) occurs by evaporation from the lungs. The percentage is dependent on the humidity of the inspired air, since the drier the air the greater the evaporative heat loss. Further, as divers go deeper and their breathing gas becomes more dense, convective heat loss increases. Breathing gas heating is needed beyond depths of 122 meters.

Symptoms of Hypothermia
On one hand, it is easy to recognise that hands and feet are cold by the familiar sensations of discomfort, numbness, pain, and diminished usefulness.

On the other hand, loss of body heat is extremely difficult to recognize. Individuals are poor judges of their own thermal state. As body heat is lost, the body approaches hypothermia; recognizing hypothermia in its early stages is a serious problem in diving. Severe hypothermia, meaning a rectal temperature of 35° C or lower, is dangerous; at this stage, a diver may become helpless.

Chilling, even if not severe enough to threaten life, will produce loss of dexterity and sense of touch in the hands, making it difficult for a diver to do useful work or even to control diving equipment such as weight belts and buoyancy compensators. Shivering causes a lack of coordination and may make it difficult for a diver to hold the mouthpiece in place. By the time shivering becomes uncontrollable, oxygen consumption has increased significantly. Before this, however, the dive should have been terminated and rewarming started.

The ability to think clearly and short-term memory also may be affected seriously by cold. When diving in cold water, it is essential for the diver to:

  • Wear thermal protection appropriate for the water temperature
  • Note the first signs of cold hands and feet and loss of dexterity and grip strength
  • Note difficulty in performing routine tasks, confusion, or a tendency to repeat tasks or procedures
  • Note feelings of being chilled followed by intermittent shivering, even though routine tasks can still be performed
  • Terminate a dive if any of the above symptoms are present
  • Be aware that even when properly dressed, hypothermia may develop without shivering
  • Watch the buddy diver and take heed of any behavioural changes that may indicate existing or approaching hypothermia.

Surface Considerations

The equipment used on the surface may be as important to divers’ comfort. A heated environment to gear up and afterwards undress can make an enormous difference not only to divers’ comfort and morale but also to keep equipment from freezing if surface temperatures are below freezing.

Large windproof coveralls may be worn over a fully dressed drysuit diver on the way to a dive site or in between dives. Proper topside hats and mitts to stay warm on the surface are essential while neoprene gloves with the fingers cut off at the knuckles may provide warmth for hands that must still require full dexterity for setting up equipment. Eating and drinking properly both before and after diving are also critical to keeping energy levels high. Diving in cold conditions also requires special emergency equipment to treat hypothermia and, as importantly, knowledge of cold weather afflictions to identify them in the early stages.

Staying Warm Underwater

Obviously, a diver exposed to cold water or even moderately warm water for long periods must wear protective clothing. Because of large individual differences in cold tolerance, every diver must determine the most suitable protection on an individual basis. A variety of diving suits is available, ranging from standard foamed neoprene wet suits and drysuit's to specially heated suits.

The use of protective equipment, however, creates a complication because the body's defence mechanism is modified by the thermal barrier of the clothing. This complication is only just being recognised as important, and divers should be aware that the faster the rate of heat loss, the smaller the drop in core temperature for a given quantity of heat loss. Furthermore, whether or not a person shivers is strongly influenced by:

  1. The rate of body heat loss
  2. The amount of body fat
  3. The body size. larger, fatter people are less affected by a given cold exposure and less affected by a given amount of heat loss. For example, because heat transfer is about 100 to 200 times faster in water than in air, the heat that reaches the skin surface is rapidly transferred to the water. Generally, the thicker the layer of subcutaneous fat, the greater the insulation.

Heat Generation

During swimming, the increase in energy production resulting from exercise is counterbalanced by the increase in muscle blood flow resulting in greater heat transfer. Thus swimming promotes faster transfer of heat from the core to the periphery, and this heat is in turn lost to the water. This is why persons suddenly immersed in cold water or divers becoming cold are better off remaining still than trying to swim.

Rapid heat loss provokes strong shivering, so that the diver is warmed. Gradual heat loss over a long time often will not cause shivering, yet the accumulated cooling and the likelihood of hypothermia may be even greater, with the likely result of impaired performance.

Staying warm

A critical factor in warmth is the clothing worn beneath the drysuit. Dry suits provide different degrees of insulation and so the optimal number of layers to maximize warmth while maintaining flexibility can be perfected over time.

  • Base Layer - The first layer will remove moisture from the body and should be a lightweight synthetic or silk long underwear. Moisture may enter as perspiration while gearing up, from exertion during the dive or from any trickle of water that may come through the drysuit. Avoid cotton and wool, which do a poor job of transporting heat and moisture, and once wet, tend to stay wet resulting in a large loss of body heat. This wicking layer should fit fairly snugly against your skin, without being restrictive. Fourth Element’s Earthier is a base layer made specifically for divers.

  • Primary Insulating Layers - The insulating layer is worn over the base layer and traps body heat while allowing perspiration to escape. This may consist of one or more layers and is typically mid weight fleece. Cheap discount brands of fleece will compress and pill and lose their insulating value and it is recommended to use name brand outdoors manufacturers.

    A long-sleeved turtleneck insulates the core and the arms, as well as the neck, an often-overlooked area of high heat loss. The turtleneck can fit under a drysuit seal without affecting the watertight seal. Feet should also have an insulating layer using either fleece or wool socks.

  • Under Suit - Both the wicking layer and insulating layer(s) provide additional insulation underneath a under suit such as those made by DUI or Weezle and most drysuit manufacturers. Adding additional layers beneath the under suit is one of the critical differences between diving in cold water and diving in extreme cold.


Dry gloves are warmer, but can be somewhat blubbery when on the surface, They also suffer from squeeze as you descend. Many divers prefer the perceived simplicity of standard wet gloves.

Regardless of how warm your body may be, if your hands get very cold, the end result is significant discomfort and possible danger as your dexterity disappears with the cold. Your hands are a vital piece of equipment and it is simply not worth skimping on insulating them properly in cold water.

Dry suits with neoprene seals will require dry gloves with latex seals that will seal under the neoprene seal. After diving with a number of drysuit gloves, the Xerotech from Helios are very popular. The heavy duty rubber gloves are lined with thick fleece and the latex wrist seals are tight but can be cut back incrementally for a custom and more comfortable fit.

There are three styles of Xerotech gloves and the usual array of sizes, Orange, Blue and Yellow and the colours reflect different thickness, Orange is the standard model, the heavy duty blue is best for diving and the super duty yellow is deigned to protect hands from cuts and punctures


In cold water, a hood is essential since 80% of body heat escapes through the head. Neoprene hoods come in different thicknesses, typically 3, 5 and 7mm, the thicker the warmer, however, any thicker and they can become uncomfortable. It is important to get a good snug fit to minimize any water flow and also it should cover as much of the head as possible without compromising masks and regulators.

Dry Suit

The most important consideration with a drysuit is to ensure that it fits properly. Fit is critical for both warmth and dexterity. The height, shoe size and wrist and neck seals must all be exact to eliminate any water trickling in and for the best insulation. Telescoping drysuit's allow people of different heights to use the same suit but using a larger shoe size or too tight a neck seal is undesirable. The drysuit should be maintained and every couple of years, depending upon use, may need seals or the zipper replaced as they wear down. There is no one particular type of drysuit that is best for extreme cold water diving. Many factors including cost, fit, strength of material if diving around areas with high abrasion, ease of repair and any potential contaminants will all affect the ideal choice. Any properly fitting drysuit with appropriate undergarments should keep a diver warm. Based purely on warmth, a neoprene suit would be the warmest but neoprene suits compress with depth and tend to lose some of their insulating qualities while also requiring more weight and generally don’t permit as much clothing underneath. Crushed neoprene solves many of these problem but isn't as warm as neoprene, though it is warmer than trilaminate or vulcanised rubber suits. Both neoprene and crushed neoprene take the longest time to dry and if surface temperatures are below freezing will freeze hard whereas rubber or trilam can be dried with a towel. We have had divers stay warm in all suits and in the end there is no one right or wrong choice.

The preferred location for the inflator is on the chest with the exhaust valve on the arm above the elbow. Exhaust valves on the ankles are unnecessary and exhaust valves placed on the forearm make leaks through the wrist seal more likely as excess air accumulates in the wrist and the valve may also interfere with dive computers or wrist slates. An exhaust valve around the bicep permits the diver’s arm to bend when exhausting air, which stops excess air from accumulating at the wrist seal.

Options on a drysuit that are helpful include a rotating inflator valve, adjustable exhaust valve, self donning zipper, relief zipper or valve, internal suspenders and possibly some type of integrated boot if walking frequently on rocks or snow. An inflator whip with a large coupler will make it easier to attach and detach while wearing dry gloves.

Full Face Masks

Full Face Masks offer some additional insulation around the face but, in our experience, are not necessary unless communication systems are desired.

Buoyancy Control Device

There are no special cold water considerations for the BC other than to ensure that it fits properly over the additional insulation. BCDs selected for diving with minimal insulation in the tropics may not be ideally suited for the additional bulk the insulation will create.

Back mounted flotation bladders (wings) with an adjustable harness are preferable over fixed size BCDs with side inflation. It is important to remember to bring BCDs in from the cold as inflator and deflator valves may freeze shut and ice crystals can damage BC bladders.


Like the BCD, the most important thing is simply to ensure that they fit properly over the drysuit with all the insulation. Large quick release buckles will make it easier to don or remove the fins while wearing dry gloves and extra straps are even more important as they tend to be more brittle in cold weather. A spring heel strap may make it easier to get fins on and off.


Cold water diving requires a significant amount of insulation and a diver will likely need a extra weight, especially if diving in salt water. It is not uncommon for a large diver to require as much as 20Kg (50Lbs). Correctly distributing this weight will make gearing up, entries and exits much easier and will affect the diver’s trim and buoyancy. Although weight harnesses such as the DUI Weight & Trim system are a nice to have, they become much more significant when carrying the enormous amount of weight necessary for extreme cold water. The harness stops the slipping and rotating of traditional belts, eliminates the risk of the weight belt loosening and evenly distributes weight to alleviate lower back stress. If using a standard weight belt, a double buckle and/or depth-compensating buckle will reduce the chance of the belt slipping off as is more common with thicker suits which may compress at depth.

The harness approach also allows for easy addition or subtraction of weights as may be necessary with changes to the number and type of undergarments worn. Distributing weight and reducing the weight carried on the belt or harness is also essential to facilitate movement, reduce stress on the lower back and make proper buoyancy easier.

There are many different ways and products to distribute weight. BCDs with weight-integrated pockets may be useful but tend to make an already heavy rig harder to handle topside. Using steel tanks versus aluminium is a quick way to reduce the lead that must be carried. Ankle weights are a good way to move 2-3Kg (3-4Lbs) off the belt. Dive Rite makes weights that clip onto the back of the BC running horizontal to the tank(s). Halcyon produces weight pockets that can be added to the BC tank strap to hold weights and many companies make V weights to put between doubles or weighted tank adapters or back plates. Regardless of exactly how you decide to reposition weights, the important thing is to spread them around and to do so in a way that you can fine tune the quantities for proper trim while keeping enough weight that can be ditched to make an emergency ascent.


In addition to proper insulation, some divers may opt to carry a pony bottle inflation system filled with Argon, a gas heavier than air. There are different opinions about the additional insulation that Argon provides over air that range from negligible to as high as 20%. The downside is an additional expense, an additional piece of equipment to maintain and the need to find an argon fill station, which may not always be widely available. Some divers swear by them and whether this is a psychological placebo effect or not is irrelevant if they feel more comfortable as a result.


Electric heaters worn under the drysuit are another specialised option available from Patco. These may be useful for people who get cold easily but are probably not otherwise essential for divers unless spending extraordinary amounts of time in extreme cold water.

Much cheaper reusable or disposable chemical heat packs such as those made by Thermo-Pad may be used for additional heat on core areas or extremities that otherwise get cold but with caution and always above at least one, if not two, layers of clothing as once under the suit it can not be removed or turned off and may burn the skin when put under pressure.

Cold Water Survival in Cold Water

Should any divers have the misfortune to find themselves isolated at sea, there are procedures that can significantly increase the chances of survival, even in extremely cold water. These include:

Keeping Warm

  • Make yourself sufficiently buoyant,
  • Inflate your drysuit as much as you comfortable can
  • Ditch weight belts, so more of is above the surface
  • Keep insulated as much as possible, leave mask on if vision is ok.
  • Do not attempt to swim except to a nearby craft, fellow survivor, or floating object. Swimming will pump out the warmed water between the body and clothing layers and cause the blood to move from the body core to the extremities, thus increasing body heat loss.
  • Keep the head and neck out of the water. The best position to conserve body heat is to hold the knees against the chest in a doubled-up fashion with the arms tight around the side of the chest. If others are nearby, huddle together and maintain maximum body contact.
  • Board a life raft or floating object as soon as possible.
  • Keep a positive attitude, because a will to live does make a difference.

Attracting Attention

  • Check torches, CDs, Mirrors and other objects for attracting attention
  • EPIRBs - If available, switch on.


At the end of a dive, a cold diver should be re-warmed. This can be accomplished by having the diver drink hot liquids such as soup or coffee, dry off in a warm place, and bathe in warm water.

Studies have shown that re-warming in 40° C water re-establishes normal body temperature 67 percent faster than re-warming in 38° C air (Strauss and Vaughan 1981).

Cold divers should not make a second dive on the same day, because it is difficult to know when body heat has been restored. However, if a second dive is necessary, it is advisable to overdo the rewarming until sweating occurs, which indicates that body heat has been restored. Exercising to generate internal heat is also helpful to speed up the rewarming process. The diver should then change into warm, dry clothing and continue some mild exercise to improve heat production and circulation. Several hours may be required to restore all the body heat lost.

Drinking alcohol is not beneficial, because it increases circulation of blood to the skin and speeds the loss of body heat in cold surroundings. A diver who is so hypothermic that he or she is helpless, irrational, or lethargic should be re-warmed more vigorously. Ideally, a warm to hot bath should be used, but if none is available, a hot water suit, electric blanket, or inhalation rewarming are suitable methods [Note - Divers who have been chilled on decompression Dives, or dives near the decompression limit, should avoid hot baths or showers because these may stimulate bubble formation. See DCS for more information]. A hypothermic diver who is helpless, irrational, lethargic, or unconscious needs medical attention and immediate and vigorous rewarming, by any of the prescribed techniques

Cold Water Regulators

Just like drysuit's, there is no clear right or wrong when selecting from high-quality cold water environmentally sealed regulators. The most important consideration will be to have the regulators serviced regularly specifically for cold water to set a lower intermediate pressure, to familiarize yourself with the adjustments you may want to make at the dive site and to follow proper cold water diving principles such as not exhaling into the regulator at the surface and ensuring the water doesn't enter the second stage between dives. Many of the top regulators meet the European standard (EN250) for cold water, see Regulators for more information.

Apeks makes a valve that sits next to the second stage allowing for immediate isolation in the event of a free flow. A pony bottle or doubles are essential when diving in cold water.

Free Flows

When a gas is compressed, a by-product of that process is heat. A recently filled cylinder is warmer than you might expect. Conversely, when air is depressurised it suffers a drop in temperature. You might have noticed that the air you inhale from your regulator is often a lot cooler than you would like.

When air from a cylinder under-goes a drop in pressure as it passes through the regulator first stage (typically cylinder pressure to 11 Bar)- with a further drop in pressure at the second stage (11 Bar to ambient pressure) it gets colder.The very cold air caused by temperature drops in each stage of the regulator will cause any water droplets within the mechanism to form ice crystals, which in turn can cause a malfunction.

Modern down-stream valves will freeze open rather than shut, but, if they freeze, a catastrophic free-flow will always be the result. Divers are trained to manage free flow situations in basic Dive training.

When a regulator suddenly goes into free-flow it can be very startling as there is a sudden roar of bubbles and visibility is reduced. The main strategy is to:

  • Remain calm Stop - Think - Act
  • If you are confident in breathing from a free flowing regulator do so, but check and prepare alternate sources just in case
  • If an alternate air supply is preferred and available switch to it.
  • Head for the surface.
  • If Buddy is available or you can reach the cylinder valve switch off cylinder and slowly switch back on.
  • If you run out of air then the only option is a free ascent, possible accellerated by removing the weight belt. Ensure you don't hold your breath and the air inside the lungs will expand as you ascend.

Breathing from a Free Flow
Hold the regulator second-stage in your mouth but angled upwards so that it is pressed against your top lip. Excess air will be free to escape from below the mouthpiece and you'll find that no water enters your mouth as you breathe.

This technique takes practice to perfect and to feel comfortable with it, so take every opportunity to simulate it in safe conditions. This can be simulated by pressing the purge button.

Preventing a Free Flow
Fresh water in Britain can often be close to freezing, even outside the obvious winter period. Ten tips for preventing regulator freeze-ups are:

  1. Be certain that your cylinder is free of moisture and has been filled with air containing as little moisture as possible.
  2. Be aware that water colder than 5°C can cause regulator icing.
  3. Keep your cylinder out of the cold until you are ready to use it. (Don't leave it in your car overnight.)
  4. Blow away any entrapped water (or ice) that may be on your cylinder valve or regulator orifices, with air from your cylinder.
  5. Ensure that even minute droplets of moisture do not enter your regulator between dives.
  6. Do not permit any water to enter the 2nd stage before or during the dive.
  7. Take check-out breaths submerged in shallow water immediately before diving rather than in the air.
  8. Avoid the cooling effect of fast air flows caused by using the purge button or breathing heavily, or filling delayed SMBs or lifting bags.
  9. Keep your 2nd stage submerged while walking through the shallows.
  10. Restrict yourself to no-stop diving, and to a depth from which you are certain you are able to make a free ascent.




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