How To Stay Warm Underwater
by John Francis
The eight best ways to beat the chill on your next dive.
You're wearing so much neoprene you can hardly breathe, let alone walk. Definitely more
than some of the other divers on the boat. Yet you're still freezing and they seem to feel
fine--they're relaxed and chatting while you're shivering. Is something wrong with your
wetsuit?
Could be. Wetsuits do wear out. Or the fault could be closer to home. Like in the mirror.
Often, when divers are cold, there's nothing wrong with the wetsuit itself. Instead, they've
chosen the wrong wetsuit, or they've made it impossible for their wetsuit to do its job. Fox
got the chickens? Don't blame the chicken coop if you left the door open.
More than discomfort is at stake. Not only is being cold while diving no fun, it is exhausting
and even a little dangerous. It's exhausting because heat is energy. Fighting a current back
to the boat and then climbing the ladder become harder when you're cold. Even though
you're able to warm up after the dive and feel comfortable again, your energy stores may be
so depleted that you pass up the next dive in favor of a lounge chair and a Tom Clancy
novel.
Getting cold during a dive is dangerous too because it can increase your risk of
decompression sickness (see "Cold and DCS"). But probably more important is the fact
that when you're uncomfortably cold you're more likely to do something stupid. As one
experienced diver told me, "It's like trying to perform a complicated and serious task while
someone is squeezing your fingers in a vise." The good news is that this problem is
fixable. There is no reason today for anyone to dive cold. There's even a chance you can be
warmer than you are now with less total neoprene, and therefore less lead around your
waist and easier buoyancy control. Here are our eight best ways to stay warm on your next
dive.
Cover Your Head
Are you wearing a hood? Nobody likes them, but head protection is essential. Your head is
the source of 20 to 40 percent of your heat loss though it accounts for only 10 percent of
your body's surface area. That's because your head is where your brain lives. Your brain
gets more than its share of your total blood flow to begin with, and much of it travels through
vessels just under your scalp, close to the cold ocean. Heat follows blood flow. When your
body gets cold, it constricts blood vessels in your arms and legs and near your skin to
conserve heat, but doesn't constrict those in your head. Those continue to carry blood--and
dump heat--at full capacity. So inch for inch, your head is more in need of thermal protection
than any other part of your body.
We don't like hoods because they restrict our head movement and often feel
claustrophobic. So compromise: Enlarge the face opening with scissors to free your chin.
Use a drysuit-style hood without a shoulder yoke for easier head movement. In warm water
use a beanie, shaped like a bathing cap with a chin strap. Because your skull is rigid
anyway, you could layer several beanies with no loss of mobility.
Cover Your Legs
Wearing only a shorty, which exposes much of your legs, is a little like forgoing a hood.
Your legs are as large a proportion of your body mass as your torso and a bigger
proportion of your surface area, and they run hotter because they demand blood flow to
work the muscles that propel your fins. Piling neoprene on your torso only is an inefficient
use of it. If you spread it out instead to cover your entire skin surface, you can get better
protection from the cold with less total neoprene.
Stop the Leaks
To the extent that a wetsuit leaks and lets the ocean flow behind the insulation, it is useless
no matter how thick it is. The biggest leak is likely to be at the collar of your wetsuit. It's hard
to make a seal against your bare neck that doesn't leak when you turn your head. And as
you swim forward, the collar tends to scoop in the ocean. The other openings, around your
ankles and wrists, don't matter as much because they are trailing behind you.
The warmest solution eliminates the collar opening entirely by attaching the hood to the
suit. But many divers find the attached-hood suit hard to get into and confining to wear. So
wetsuit makers have gotten better at designing neck seals with smooth inner surfaces that
adjust to lie flat against your skin. Pay some attention to the collar adjustment behind your
head when you dress. A buddy can be a big help here, since it's hard to see or reach that
adjustment tab yourself.
Often the bulky neck and shoulders section of a hood (the "yoke") improves the collar seal
just by making it tighter. That alone can be reason enough to wear a hood. Some divers cut
the head of a hood and wear the yoke only, with a drysuit-style hood or a beanie. That can
give a better collar seal, good head protection and more mobility.
Get a Suit that Fits
Even if all your wetsuit's openings seal perfectly, it will still leak if it doesn't fit well. The
best-fitting wetsuit would be molded on your body and glued to your skin so that when you
move, it moves. But in reality there are bound to be some pockets and wrinkles--in your
armpits and groin, behind your knees and in front of your elbows. Whenever you move an
arm or leg, those pockets and wrinkles expand and collapse. When they do that, they
become pumps, sucking water in and squirting it out. If cold ocean water is sucked in
against your skin, it steals your body heat. When it's squirted out into the ocean, it carries
your heat with it. That's why a spine pad can make a wetsuit warmer--not because it adds
more insulation but because it fills that hollow along your spine that would otherwise be a
water pump. Yes, seals can stop the flushing; that makes dry suits possible. But some
seals are better than others, and the best can be defeated. Why put them to the test?
A good fit is pretty snug. In fact, if a wetsuit is easy to pull on, it's probably too loose. You
need to be able to breathe and move, of course. But short of that point, the tighter the better.
Your wetsuit should fit equally tight all over, because any areas looser than others are likely
to become those water pumps when you flex arms and legs.
Getting a snug fit in your armpits, groin and behind your elbows is especially difficult. Be
sure you pull it up as far as you can.
Go Custom
A custom-measured wetsuit should fit best. One custom wetsuit maker we know, for
example, takes six circumference measurements and two length measurements of each
leg alone. Among off-the-rack suits, look for brands that offer the most size options,
including long and short versions. The more sizes you can choose from, the better chance
of a close, warm fit. If you put fit ahead of other features, you'll be glad later.
Start Your Dive Warm
Many divers squander their body heat before they get wet, then blame their wetsuit when it's
gone. Gradual heat loss, feeling a little cool but not cold, goes unnoticed but adds up.
Nobody travels to the tropics so they can wear a jacket, but if you dress a little too warm
instead of a little too cool before and after you dive, you'll take more heat with you into the
water.
Your biggest heat loss while out of the water is likely to be during the surface interval.
You're wet, and as the water evaporates it sucks heat out of your body. Any wind
accelerates the process. It's not unusual to get colder in the air than you were in the water,
which you already know if you've ever rushed your surface interval to get back in and warm
up. The solution is to undress and towel off during your surface interval, or at least wipe
down the outside of your wetsuit and put on a dive coat or even a raincoat to stop the wind,
which is the swamp cooler's fan.
Stay Shallow
Deep water tends to be colder anyway, but the greater pressure is a bigger problem. Your
wetsuit loses insulation with every foot of descent because the insulating bubbles in the
neoprene, like all bubbles, compress under pressure. At only 33 feet, the pressure on
those bubbles has doubled to two atmospheres, taking nearly half the insulation from your
wetsuit. ("Nearly" because the neoprene itself has a little resistance to compression.) By 99
feet, or four atmospheres, the bubbles are essentially flat and your wetsuit has practically
no insulation left.
Of course, if the wreck is at 99 feet, that's where you have to go. But divers often go deep for
no real reason. A wall often looks the same at 40 feet as at 80 feet, for example, so why not
explore the shallows when the water is cold? Or make a transit across the top of the reef to
the wall, or out to the descent line to the wreck, as shallow as possible?
If a lot of your diving is in deep and cold--or not so cold--water, it's time to consider a dry
suit. The insulating air in a dry suit compresses with depth too, of course, but you can inject
more to reinflate the suit and restore the warmth.
Buy New
If you think you're doing everything right and you're still cold despite wearing what ought to
be enough neoprene, maybe there is something wrong with your wetsuit. They do wear out,
and the wear can be invisible--from the inside out. It's the bubbles inside the neoprene that
matter, remember, and when they flatten during a deep dive, some of them don't regain
their full size and insulation value.
Even in storage, those bubbles eventually break down as the neoprene is attacked by
ultraviolet radiation and by chemicals in the air--the exhaust fumes in your garage, for
example. Storing your wetsuit with weights and tanks on top of it is worse, like subjecting it
to a dive that lasts for weeks. In time, all bubbles die.
How much time? It depends on how deep you dive, how often, how carefully you store your
wetsuit and on the quality of the neoprene to begin with. Generally, three to five years is the
useful lifetime of a wetsuit. At the end of its life it can still look perfect, but it won't keep you
warm. When you go shopping for a new one, you know what to look for: full coverage, a
leak-proof collar seal and an even, snug fit.
Cold & DCS
Most dive tables and algorithms assume your blood flows at the same rate throughout your
dive, in the descent phase when you're taking on nitrogen and in the ascent phase when
you're offgassing. In fact, if you get cold, it's usually in the later ascent phase, which causes
your body to constrict many blood vessels, especially in your arms and legs. The result:
Your blood moves faster when it's delivering nitrogen to your tissues and slower when it's
taking nitrogen away. When you get cold, you offgas more slowly than your computer
predicts, so you carry a larger nitrogen load and a greater DCS risk than you realize.
How a Wetsuit Really works
It's all in the bubbles. Contrary to myth, the layer of warm water trapped inside your wetsuit
does not keep you warm. Water has little or no insulation value and huge chilling power if it
flows out of your suit, carrying heat with it. It's a necessary evil you'd be better off without.
Insulation is a barrier--really, a gap that heat has a hard time jumping across when trying to
get from where it is concentrated (a high temperature) to where it's not (a low temperature).
The bigger the gap, the slower the flow of heat, the better the insulation.
We tend to think of insulation as a substance, like styrofoam or neoprene, but the best
insulation is nothing, a vacuum. Next-best is a gas. It's the gas in the bubbles trapped in
the styrofoam or neoprene, not the plastic or rubber itself, that blocks the flow of heat.
Do You Need a Wetsuit in the Tropics?
Yes. Immersion in water cooler than about 90 degrees will still pull heat from your body. In,
say, 82-degree water your heat loss will be so gradual that you hardly notice it. The water
feels warm when you get in it, and even after an hour you're probably not really
uncomfortable. But you have lost heat and therefore energy. You're now thinking slower and
responding slower to conditions. This gradual heat loss is likely to accumulate over several
days of diving until by the fourth or fifth day you're passing up dives to lie in the sun because
you feel tired, not cold. And is that what you flew thousands of miles to do?
How Much Is Enough?
Assuming full coverage and a good fit, how thick does your wetsuit need to be to keep you
warm?
It's hard to generalize because sensitivity to cold differs so much from diver to diver.
Children and older divers often need more thickness, while overweight divers, having more
natural insulation, may need less of the artificial kind. But here's a rough guideline:
WATER TEMPERATURE* COVERAGE
75-85 F Skinsuit or 1mm Neoprene
70-80 F 3mm Neoprene
65-75 F 5mm Neoprene
50-70 F 7mm Neoprene
Below 55 F Dry Suit
* Depth matters too. Below 60 feet, consider the next-higher level of protection.
by John Francis
The eight best ways to beat the chill on your next dive.
You're wearing so much neoprene you can hardly breathe, let alone walk. Definitely more
than some of the other divers on the boat. Yet you're still freezing and they seem to feel
fine--they're relaxed and chatting while you're shivering. Is something wrong with your
wetsuit?
Could be. Wetsuits do wear out. Or the fault could be closer to home. Like in the mirror.
Often, when divers are cold, there's nothing wrong with the wetsuit itself. Instead, they've
chosen the wrong wetsuit, or they've made it impossible for their wetsuit to do its job. Fox
got the chickens? Don't blame the chicken coop if you left the door open.
More than discomfort is at stake. Not only is being cold while diving no fun, it is exhausting
and even a little dangerous. It's exhausting because heat is energy. Fighting a current back
to the boat and then climbing the ladder become harder when you're cold. Even though
you're able to warm up after the dive and feel comfortable again, your energy stores may be
so depleted that you pass up the next dive in favor of a lounge chair and a Tom Clancy
novel.
Getting cold during a dive is dangerous too because it can increase your risk of
decompression sickness (see "Cold and DCS"). But probably more important is the fact
that when you're uncomfortably cold you're more likely to do something stupid. As one
experienced diver told me, "It's like trying to perform a complicated and serious task while
someone is squeezing your fingers in a vise." The good news is that this problem is
fixable. There is no reason today for anyone to dive cold. There's even a chance you can be
warmer than you are now with less total neoprene, and therefore less lead around your
waist and easier buoyancy control. Here are our eight best ways to stay warm on your next
dive.
Cover Your Head
Are you wearing a hood? Nobody likes them, but head protection is essential. Your head is
the source of 20 to 40 percent of your heat loss though it accounts for only 10 percent of
your body's surface area. That's because your head is where your brain lives. Your brain
gets more than its share of your total blood flow to begin with, and much of it travels through
vessels just under your scalp, close to the cold ocean. Heat follows blood flow. When your
body gets cold, it constricts blood vessels in your arms and legs and near your skin to
conserve heat, but doesn't constrict those in your head. Those continue to carry blood--and
dump heat--at full capacity. So inch for inch, your head is more in need of thermal protection
than any other part of your body.
We don't like hoods because they restrict our head movement and often feel
claustrophobic. So compromise: Enlarge the face opening with scissors to free your chin.
Use a drysuit-style hood without a shoulder yoke for easier head movement. In warm water
use a beanie, shaped like a bathing cap with a chin strap. Because your skull is rigid
anyway, you could layer several beanies with no loss of mobility.
Cover Your Legs
Wearing only a shorty, which exposes much of your legs, is a little like forgoing a hood.
Your legs are as large a proportion of your body mass as your torso and a bigger
proportion of your surface area, and they run hotter because they demand blood flow to
work the muscles that propel your fins. Piling neoprene on your torso only is an inefficient
use of it. If you spread it out instead to cover your entire skin surface, you can get better
protection from the cold with less total neoprene.
Stop the Leaks
To the extent that a wetsuit leaks and lets the ocean flow behind the insulation, it is useless
no matter how thick it is. The biggest leak is likely to be at the collar of your wetsuit. It's hard
to make a seal against your bare neck that doesn't leak when you turn your head. And as
you swim forward, the collar tends to scoop in the ocean. The other openings, around your
ankles and wrists, don't matter as much because they are trailing behind you.
The warmest solution eliminates the collar opening entirely by attaching the hood to the
suit. But many divers find the attached-hood suit hard to get into and confining to wear. So
wetsuit makers have gotten better at designing neck seals with smooth inner surfaces that
adjust to lie flat against your skin. Pay some attention to the collar adjustment behind your
head when you dress. A buddy can be a big help here, since it's hard to see or reach that
adjustment tab yourself.
Often the bulky neck and shoulders section of a hood (the "yoke") improves the collar seal
just by making it tighter. That alone can be reason enough to wear a hood. Some divers cut
the head of a hood and wear the yoke only, with a drysuit-style hood or a beanie. That can
give a better collar seal, good head protection and more mobility.
Get a Suit that Fits
Even if all your wetsuit's openings seal perfectly, it will still leak if it doesn't fit well. The
best-fitting wetsuit would be molded on your body and glued to your skin so that when you
move, it moves. But in reality there are bound to be some pockets and wrinkles--in your
armpits and groin, behind your knees and in front of your elbows. Whenever you move an
arm or leg, those pockets and wrinkles expand and collapse. When they do that, they
become pumps, sucking water in and squirting it out. If cold ocean water is sucked in
against your skin, it steals your body heat. When it's squirted out into the ocean, it carries
your heat with it. That's why a spine pad can make a wetsuit warmer--not because it adds
more insulation but because it fills that hollow along your spine that would otherwise be a
water pump. Yes, seals can stop the flushing; that makes dry suits possible. But some
seals are better than others, and the best can be defeated. Why put them to the test?
A good fit is pretty snug. In fact, if a wetsuit is easy to pull on, it's probably too loose. You
need to be able to breathe and move, of course. But short of that point, the tighter the better.
Your wetsuit should fit equally tight all over, because any areas looser than others are likely
to become those water pumps when you flex arms and legs.
Getting a snug fit in your armpits, groin and behind your elbows is especially difficult. Be
sure you pull it up as far as you can.
Go Custom
A custom-measured wetsuit should fit best. One custom wetsuit maker we know, for
example, takes six circumference measurements and two length measurements of each
leg alone. Among off-the-rack suits, look for brands that offer the most size options,
including long and short versions. The more sizes you can choose from, the better chance
of a close, warm fit. If you put fit ahead of other features, you'll be glad later.
Start Your Dive Warm
Many divers squander their body heat before they get wet, then blame their wetsuit when it's
gone. Gradual heat loss, feeling a little cool but not cold, goes unnoticed but adds up.
Nobody travels to the tropics so they can wear a jacket, but if you dress a little too warm
instead of a little too cool before and after you dive, you'll take more heat with you into the
water.
Your biggest heat loss while out of the water is likely to be during the surface interval.
You're wet, and as the water evaporates it sucks heat out of your body. Any wind
accelerates the process. It's not unusual to get colder in the air than you were in the water,
which you already know if you've ever rushed your surface interval to get back in and warm
up. The solution is to undress and towel off during your surface interval, or at least wipe
down the outside of your wetsuit and put on a dive coat or even a raincoat to stop the wind,
which is the swamp cooler's fan.
Stay Shallow
Deep water tends to be colder anyway, but the greater pressure is a bigger problem. Your
wetsuit loses insulation with every foot of descent because the insulating bubbles in the
neoprene, like all bubbles, compress under pressure. At only 33 feet, the pressure on
those bubbles has doubled to two atmospheres, taking nearly half the insulation from your
wetsuit. ("Nearly" because the neoprene itself has a little resistance to compression.) By 99
feet, or four atmospheres, the bubbles are essentially flat and your wetsuit has practically
no insulation left.
Of course, if the wreck is at 99 feet, that's where you have to go. But divers often go deep for
no real reason. A wall often looks the same at 40 feet as at 80 feet, for example, so why not
explore the shallows when the water is cold? Or make a transit across the top of the reef to
the wall, or out to the descent line to the wreck, as shallow as possible?
If a lot of your diving is in deep and cold--or not so cold--water, it's time to consider a dry
suit. The insulating air in a dry suit compresses with depth too, of course, but you can inject
more to reinflate the suit and restore the warmth.
Buy New
If you think you're doing everything right and you're still cold despite wearing what ought to
be enough neoprene, maybe there is something wrong with your wetsuit. They do wear out,
and the wear can be invisible--from the inside out. It's the bubbles inside the neoprene that
matter, remember, and when they flatten during a deep dive, some of them don't regain
their full size and insulation value.
Even in storage, those bubbles eventually break down as the neoprene is attacked by
ultraviolet radiation and by chemicals in the air--the exhaust fumes in your garage, for
example. Storing your wetsuit with weights and tanks on top of it is worse, like subjecting it
to a dive that lasts for weeks. In time, all bubbles die.
How much time? It depends on how deep you dive, how often, how carefully you store your
wetsuit and on the quality of the neoprene to begin with. Generally, three to five years is the
useful lifetime of a wetsuit. At the end of its life it can still look perfect, but it won't keep you
warm. When you go shopping for a new one, you know what to look for: full coverage, a
leak-proof collar seal and an even, snug fit.
Cold & DCS
Most dive tables and algorithms assume your blood flows at the same rate throughout your
dive, in the descent phase when you're taking on nitrogen and in the ascent phase when
you're offgassing. In fact, if you get cold, it's usually in the later ascent phase, which causes
your body to constrict many blood vessels, especially in your arms and legs. The result:
Your blood moves faster when it's delivering nitrogen to your tissues and slower when it's
taking nitrogen away. When you get cold, you offgas more slowly than your computer
predicts, so you carry a larger nitrogen load and a greater DCS risk than you realize.
How a Wetsuit Really works
It's all in the bubbles. Contrary to myth, the layer of warm water trapped inside your wetsuit
does not keep you warm. Water has little or no insulation value and huge chilling power if it
flows out of your suit, carrying heat with it. It's a necessary evil you'd be better off without.
Insulation is a barrier--really, a gap that heat has a hard time jumping across when trying to
get from where it is concentrated (a high temperature) to where it's not (a low temperature).
The bigger the gap, the slower the flow of heat, the better the insulation.
We tend to think of insulation as a substance, like styrofoam or neoprene, but the best
insulation is nothing, a vacuum. Next-best is a gas. It's the gas in the bubbles trapped in
the styrofoam or neoprene, not the plastic or rubber itself, that blocks the flow of heat.
Do You Need a Wetsuit in the Tropics?
Yes. Immersion in water cooler than about 90 degrees will still pull heat from your body. In,
say, 82-degree water your heat loss will be so gradual that you hardly notice it. The water
feels warm when you get in it, and even after an hour you're probably not really
uncomfortable. But you have lost heat and therefore energy. You're now thinking slower and
responding slower to conditions. This gradual heat loss is likely to accumulate over several
days of diving until by the fourth or fifth day you're passing up dives to lie in the sun because
you feel tired, not cold. And is that what you flew thousands of miles to do?
How Much Is Enough?
Assuming full coverage and a good fit, how thick does your wetsuit need to be to keep you
warm?
It's hard to generalize because sensitivity to cold differs so much from diver to diver.
Children and older divers often need more thickness, while overweight divers, having more
natural insulation, may need less of the artificial kind. But here's a rough guideline:
WATER TEMPERATURE* COVERAGE
75-85 F Skinsuit or 1mm Neoprene
70-80 F 3mm Neoprene
65-75 F 5mm Neoprene
50-70 F 7mm Neoprene
Below 55 F Dry Suit
* Depth matters too. Below 60 feet, consider the next-higher level of protection.
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