by Chris Woodford. Last updated: January 6, 2017.
Everyone loves fresh, clean clothes—but how many of us enjoy the chore of
getting them that way? Not many! Why? Because the process of cleaning
clothes with water and detergent is inefficient, laborious, and time
consuming. Clothes washing machines do a superb job of rinsing away
the sweat and dirt our garments pick up each day, typically in less
than an hour, but they have one mighty drawback: they give you back
your clothes clean but soaking wet! Before you can put your t-shirt
and jeans back on, you need to get rid of a huge amount of water,
either by indoor or outdoor drying. What's the best way to do that?
Let's look at the science and find out!
Photo: Indoor drying: A typical tumble dryer in a laundry uses natural gas burners to dry your clothes inside a very large rotating drum. The burners, located at the bottom of the machine, create a steady updraft of hot rising air that floats up through hundreds of small holes in the drum. Paddles (vertical bars mounted around the inside of the drum) constantly toss and tumble your clothes so they repeatedly fall through the hot air. Photo by Walter M. Wayman courtesy of US Navy.
The science of drying clothes
If you understand a little bit of the science behind drying your
clothes, you'll find you can do your laundry a whole lot more quickly and
economically. So what exactly is involved in turning wet clothes back
into dry ones? In a word, evaporation: turning the liquid water in your clothes into a
vapor (gas)—and then getting rid of it.
Photo: Right: Outdoor drying: When you hang socks outside to dry, you're using energy from the Sun and the wind to evaporate and disperse the water they contain. It's free to dry your clothes this way and you'll find it helps them last longer compared to using a tumble dryer. Since you're not using electricity to get things dry, it's better for the
How much water is in wet clothes?
Water is the world's most versatile cleaner, but it's amazing how much of it
you can waste when you do the laundry. On its shortest and simplest
setting, with only a half load of washing, my old clothes washing
machine uses about 20 liters (5 gallons) of water; with multiple rinses, many
machines will easily use double that much.
Most of that water is spun out at high speed (think centrifuge)
and then drained away, but even the most efficient machines leave a
significant amount of wetness lingering in your clothes. While
writing an excellent book about how people can use energy more
efficiently, physicist David MacKay weighed his laundry and discovered
that a typical 4kg (8.8lb) load of dry washing emerged 2.2kg (4.9lb) heavier, even
after vigorous spinning in a modern machine
(read more in his discussion of
home heating and cooling). So the price you pay for
clean clothes is wet clothes that are at least 50 percent
heavier than they were when you loaded them into the machine.
Before you can wear those clothes, or put them safely away in your cupboard,
you have to get rid of that water. So the everyday chore of drying
clothes is actually a more scientific kind of problem: how can we
dispose of at least 2kg (4.9lb) of water as quickly and efficiently as
possible? If you bear in mind that 1kg (2.2lb) of water is roughly 1 litre (2.1 US pints),
you can picture the problem very vividly: for every load of wet
washing, what you have to do is make the equivalent of at least two
full liter bottles of water (just over four pints) literally vanish
into thin air. That's actually quite a tall order—and it's probably
rather more water than you imagine your clothes containing. Dry your
clothes inside your home (without an electric dryer) and what you're
doing is throwing about a quarter of a bucket of water into the air
in extremely slow motion. Think about that the next time you
hang your wet socks to dry on your bedroom radiator. Would you really
want to hurl a quarter of a bucket of water in there?
Photo: Two liters of water: This is how much you'll find in your typical 4kg load of washing when you remove it from your clothes washing machine. It's what you have to get rid of before your clothes are properly dry. If you dry your clothes indoors on radiators or a clothes rack, this water will end up floating around inside your home. (Pen in front indicates scale.)
How can you remove the water from your clothes?
The simplest way of getting rid of liquid water is to turn it into a
vapor (broadly speaking, that means a gas produced from a
liquid)—and the easiest way to do that is to heat it up. The
molecules in a liquid are closer and more tightly bound together, move
more slowly, and have less energy than the molecules in a gas.
(This idea is part of the kinetic theory of matter—a way of
understanding how solids, liquids, and gases behave by thinking about
the molecules inside them buzzing about in constant motion.) If you want
to turn a liquid into a gas, you need to put in quite a bit of energy
so the liquid molecules can break apart, escape from the bulk of the
liquid, and form a vapor above it. Putting heat into a liquid is an
easy way to achieve this. Heat a liquid and you make the molecules,
on average, more energetic so they have more chance to escape. Heat
it enough and all the molecules will eventually evaporate—in theory,
at least—leaving you with no liquid at all. So one way to dry wet clothes is to heat them up, turn the
water they contain into steam, and then extract the steam so dry
clothes are left behind. That's roughly how a tumble clothes dryer machine
works, as we'll discover in a moment.
Turning liquids into gases sounds like the stuff of chemistry labs, but it's
exactly the sort of science we all dabble in every day. Whenever you
boil a kettle, you convert cold liquid water into hot boiling water
and—unless you turn the kettle off promptly—an astonishing
amount of hot water vapor, commonly known as steam. Bear in
mind that steam at a temperature of 100°C occupies roughly 1500 times as
much room as liquid water and you'll understand why drying clothes
(or cooking) can very quickly generate a massive amount of hot, humid
air (saturated with water).
Producing steam from water requires a huge amount of energy, known as the
latent heat of vaporization (you can read more about this
in our article on states of matter). How much energy, exactly? Suppose
the wet clothes from your washing machine contain 2kg (4.9lb) of water at
15°C (59°F). To get rid of that water, let's say you use an electric
tumble dryer, which first heats the water from 15°C to 100°C (from 59°F to 212°F),
using about 700kJ of energy, and then turns it into 2 kg of steam at
the same temperature, using another 4500kJ of energy and making about
5200kJ (1.4 kilowatt hours) of energy in total. That's about as much as running an
energy-saving lamp continually for 6 days and nights.
Electric dryers are expensive to run, not so much because they're inefficient, but
because of the basic physics involved: it takes lots of energy to
turn cold water into hot steam—and there's really no way around that.
(In reality, the energy you need is quite a bit more than I've estimated here because you're heating the clothes as well as
the water, rotating them constantly in a drum, and losing energy all the time in various ways.)
Can you dry clothes outside on cold days?
In a word, yes—but let me prove it to you, in theory and in practice.
The easiest way to get rid of water is to turn it from a liquid to a gas. If you look at the diagram on the right, you can see there are two ways to do this. One method is to use evaporation, in which the liquid turns directly to gas. You can also get from liquid to gas by making a solid first. For clothes drying, that means freezing your clothes (drying them on a winter day) and then letting the ice turn to gas (by sublimation). That's a very laborious way to dry your clothes but it does work—at least in theory!
The diagram on the left is what we call a phase diagram: it's a simple way of showing
how a particular substance will be either a solid, liquid, or gas depending on the temperature and pressure. For
water, the phase diagram is a little bit more complex than the one I've shown here, but let's assume this one is broadly correct.
At high temperatures, you can see that water is going to be gas (steam) unless the pressure is high too; at low temperatures, water is generally going to be ice, unless the pressure is low. The diagram also shows us that you can heat solid ice or liquid water to make water vapor (gas) and squeeze (increase the pressure on) liquid water or water vapor to make ice.
Ice, water, and steam can all coexist at a certain temperature and pressure known as the triple point of water (shown by the green dot in the center). The magic temperature is 0.01°C, which proves (contrary to what many people believe) that you can happily dry clothes outside even in winter, providing the humidity is low (in other words, in a cool, dry wind). Assuming you start off at the same temperature, it's always going to require the same amount of energy to evaporate the water from your clothes, whichever drying method you use. On a cool day (when the rate of energy input from the Sun and wind is lower) it will obviously take much longer to get things dry. But a dry wind will still blow water away from your clothes, even if it does take all day.
Don't let the idea of "cold" put you off. From a scientific point of view, a "cold" day is only relatively cold: even when it's 0°C, that's still 273 degrees above absolute zero—the real definition of cold, the point where atoms and molecules stop moving—and there may well be enough energy around outside to dry your clothes. What's more, a "cold" day is not necessarily as cold as you think. If the sun is shining, there may be areas outside your home where the local temperature is significantly higher than the average—in a dry, sheltered courtyard, for example. Also, if the sun is shining directly onto your clothes, that will certainly evaporate water more quickly.
Now you might say "That's a load of baloney! I don't believe you!" But this is science and
what you or I believe isn't what counts. Ultimately, what matters in science
is what there's evidence for—so let's collect some and put the theory to the test.
On December 1, 2010, a day when a winter freeze was
causing chaos throughout the UK, I decided to do an experiment with my washing.
Where I live, there was no snow, but there was no sun either: it was just very cold (round about 0–1°C) and
there was a bitterly cold, very dry easterly wind (according to the local weather forecast, 21–50km/h or 13–31mph). I did a small load of washing and weighed it when it emerged from the machine at roughly 5kg (11lb). I hung it outside for about four and a half hours, brought it in, and then put it back on the scales, discovering to my delight it now weighed only 3.5kg (7.7lb). The clothes weren't completely dry, but the cold wind had unquestionably removed a significant amount of water. After more drying indoors, I put the clothes back on the scales a third time and found they now weighed in at 3kg (6.6lb). Assuming they were completely dry at that point, we can see the wash added 2kg (4.4lb) to their weight, and outdoor drying removed 75 percent of the water. Pretty impressive for a winter's day!
Where does the water go?
If plants could talk, they'd tell you you don't have to heat water to
its boiling point to make it evaporate. A plant can turn liquid water
into water vapor at relatively low temperatures. When wind blows past
trees, for example, water evaporates from their leaves and turns into
cool water vapor that dissipates in the air. The more the wind blows,
the more water plants lose by this process, which is known as
transpiration, and the more you need to water them.
The reverse is true also. If the air is still, humid air can accumulate
near the ground around plants, reducing the amount of water that they
lose. Broadly speaking, blowing air past wet things means that water
will evaporate from them more quickly, cooling them down in the
process. And that's a very useful thing to know when it comes to
drying your clothes. If you want them to dry properly, the water they
contain doesn't just need to turn to a vapor; it has to be completely
removed from the air around them. If water vapor lingers near
your clothes, it'll not only hinder more liquid water from escaping,
but some of the molecules in the vapor will also reenter your clothes
and turn back into liquid, wetting them again! That's why hanging
your clothes outside on damp days (when it's rained recently, or when
there's low cloud or mist) is a waste of time: even if it's warm, the
high level of humidity in the air will make it difficult for water to
evaporate from your clothes and it's very unlikely they'll dry. They may even pick up moisture and get wetter!
What are the perfect conditions for drying clothes?
To sum up, we've discovered that three things favor the evaporation of water
from wet clothes:
- High temperatures—to increase the number of molecules that can turn from liquid to vapor.
- Air movements—to carry water away and prevent the air near your clothes from
becoming saturated with vapor.
- Low humidity—so evaporation will continue steadily and water molecules won't return to your clothes
from the air.
Now we've got the science nailed, let's compare the
various different ways of drying clothes outdoors and indoors.
Outdoor drying (sometimes called air-drying) with a clothes line or rotary
dryer has several big advantages: it's free, it uses no energy (so
it's environmentally friendly), it generally leaves your clothes
smelling fresh, and it means you don't make your home damp and cold
by drying things inside. The drawbacks include the time taken to dry
things (which can range from a few hours to a day or more), the chance
of rain making your clothes wetter than they were when you hung them
out, the risk of theft, and the possibility of air pollution making
your clothes dirty again.
The science we've already considered tells us that the best conditions for
outdoor drying will be warm, windy days when the humidity is
relatively low. Since we want air to move around our clothes, it's
best to dry them off the ground where the air moves faster (which
also helps to lift them away from any ground-level moisture).
Although hot, breezy, summer days are perfect for outdoor drying, you
may find you can dry things outside for much of the year, if you keep
science in mind (it depends to a large extent on the climate where
you live). Water can turn from a liquid into a vapor at any temperature
above freezing. In other words, providing the air isn't damp, you can
theoretically dry clothes outside any day of the year. Living near
the sea with the benefit of dry winter sea breezes, I can sometimes
dry huge sheets outside even in the middle of winter (or get them
half or two thirds dry at least).
Photo: Left: If you're drying with a washing line, prop your clothes up high so they're above ground level moisture and benefit from higher wind speed. Right: A rotary clothes dryer like this uses space more efficiently than a traditional clothes line, but it might take longer to dry clothes if they're hung closer together and create higher humidity as a result. If you load a dryer like this properly, it should spin in the wind, which will help to dissipate water vapor.
In theory, you can even dry things outdoors in the very depths of
winter. Instead of the liquid water evaporating and turning into a
gas, it will cool down and turn to ice, which will then very slowly
turn directly to a gas by the process of sublimation. If you
want to dry clothes this way, you need to be incredibly patient and
quite prepared to leave them outdoors for days until all the liquid water
has turned to water vapor. If you let your clothes freeze solid and
then bring them back indoors before they're dry, you're probably
going to use more energy than if you'd simply dried them
indoors to start with, because you'll have to heat them up to thaw
them out (turning the ice back to water) and then dry them
conventionally as well.
If you remember only one thing from this article, make it this sentence:
indoor drying may seem convenient, but it's a waste of money, harms
the environment by wasting energy, slowly ruins your clothes
(wrecking elastic and destroying things like t-shirt transfers), and
risks damaging your home and your health by making your indoor
environment damp and unpleasant. All these problems stem from two
issues that follow directly from the science we've already explored.
Photo: Inside a typical electric tumble dryer. Hot air enters through the holes at the back. Paddles mounted inside the drum (the gray line on the left) lift and tumble the clothes. Moist exhaust air exits through the lint filter at the bottom of the drum in front.
First, no matter how you dry clothes, you have to put in energy from
somewhere to evaporate the water. Dry things outside and that energy
comes for free from the Sun and the wind. Dry things on indoor
radiators and the energy comes from your stove, gas boiler, or heating system. The laws of
physics tell us that you cannot dry clothes for free indoors:
the energy has to come from somewhere. You'll pay more to heat your
home if you routinely dry things on radiators than if you don't. Even
if your heating is on anyway, you'll need to have it on for longer
(or turned up higher) to maintain the same temperature in your home
and dry your clothes. If you dry clothes on a rack indoors,
the energy needed to evaporate the water comes from the ambient
(surrounding) air in your home—so your home is cooling down slightly
to dry your clothes and costing you more that way. If you have no
alternative to drying clothes indoors, you may find a dehumidifier is
a good investment: by removing water vapor from the air, it makes
your home healthier and can help to reduce heating costs.
The second problem with indoor drying is that the water you evaporate has
to go somewhere. With outdoor drying, this isn't a problem: the
water simply dissipates in the air. Indoors, you'll find the steam or
water vapor you create quickly appearing on your windows as
condensation or (worse) as damp or mold on the walls. Remember how
much water we're talking about: drying a typical load of 4kg (8.8lb) will add
about 2 liters (about 4 US pints) of water to the inside of your home! Open the windows
and you'll get rid of the water vapor, but you'll also lose all the
heat energy the water contains (and some of the heat locked inside your building as well), so your home will cool down in the
process—which isn't good in the middle of winter. If you dry with an
electric tumble dryer, water vapor in your home isn't usually a
problem: most dryers either vent the damp air out through a hose or
condense it back to water that drips away down the drain. But the
drawback is the huge amount of electricity they use.
Photo: Quick dry: The biggest advantage of using an electric tumble clothes dryer is that it gets things dry much faster than almost any other drying method. These are typical drying times written on the case of a modern dryer made by the German company Bosch. According to this, even a big 6kg load would dry within a couple of hours, which is much faster than you'd manage outdoors in many countries.
How does a tumble dryer work?
We've seen from the science that the best drying conditions involve heat,
air movement, and low humidity (or the constant removal of moisture). Clothes drying machines
(also called tumble dryers) combine these things to dry clothes
quickly and efficiently inside a large, rotating metal drum.
Unfortunately, dryers like this cost a lot of money to run and they can shrink,
damage, or slowly degrade your clothes (check washing labels on your
clothes very carefully before putting them in a dryer like
The basic idea is to blow hot dry air into one side of the drum as it tumbles the
clothes around and extract moist wet steam from another part of the drum at the same time.
Step-by-step, here's how it works. (In this picture, we're looking from the side and the
front of the machine is on the left.)
- The heart of the machine is a large metal drum with paddles around its inside rim (shown here as horizontal gray lines). In large machines, such as those in launderettes, the drum always rotates in the same direction. In smaller home machines, the drum rotates one way for maybe 30 seconds or so, then stops, then rotates the other way to stop your clothes bunching up.
- Cold air is drawn into the machine through an air intake. Often it's at the front of the machine to stop it getting dirty and dusty (as it would around the back).
- A fan sucks the air in and pulls it toward a heating element.
- The fan is powered by an electric motor (read more about how motors work).
- As cool air passes over the heating element, it's warmed and turned to hot dry air. A
thermostat (not shown) turns the heating element on and off periodically to stop the machine overheating or cooking your clothes. When you select either the low or high temperature setting on your dryer, you're effectively altering the thermostat setting.
- Warm air from the heating element enters the drum, typically through large holes at the back. In launderette machines, the entire drum is full of small holes and hot air rises up from below. (The photo at the very top of this page shows a large laundry dryer like this.)
- The drum is rotated slowly by a belt connected to the electric motor, made from something like rubber. Often, as shown here, one electric motor drives both the drum and the fan.
- As the drum rotates, the paddles lift and tumble your wet clothes until they reach the top of the drum. Then gravity makes them fall back down through the hot, dry air. Dryers work most efficiently when the washing tumbles through the hot air this way. If you overload them, the washing just bunches up and rolls around in a big ball instead of tumbling and it takes much longer to dry.
- The air that leaves the dryer passes through a lint filter that catches dust and bits of fluff. Some dryers have a second (exhaust) fan to help extract the moist air. To avoid fires, it's essential to clean the lint filter in a dryer regularly (ideally, every time you used it).
- Exhausted air passes up through a vent hose either mounted permanently in the ceiling (as here) or temporarily poked through an open window. In some dryers, and most combined washer-dryer machines, the humid exhaust air is passed through a heat exchanger and condenser so the water is cooled and drained away and the heat it contains is captured and reused, making the whole process more efficient.
So what's the best way to dry clothes?
- Air dry outside if you possibly can. It's better for your
pocket, your clothes, your home, your health, and the planet.
- Choose your days carefully. Depending on the climate where you live,
humidity may well be a more important factor than temperature. In
other words, you might find you can dry things outside even on cold dry days
if it's not too damp. Check out a weather website that gives a humidity forecast
where you live.
I find outdoor drying works best for humidities less than about 80
percent, really well for 75 percent or lower, and not at all if the humidity
is over 90 percent.
- On dry winter days, humidity tends to fall to a minimum in the middle of the day.
Hang things out mid-morning (after any mist or dew has disappeared) and
bring them in before any evening dampness returns.
- The weather or time of year may prevent you drying things completely
outdoors, but it's still well worth trying to get things half or two
thirds dry that way. If you half dry outside and then tumble dry to
finish, you'll still cut your dryer bills in half—and your clothes
will last longer.
- If you must dry things indoors, the best way to do it is with an airing
cupboard (the cupboard that surrounds your hot water tank, if you
have one). If your home suffers from damp, routinely drying things
indoors is a bad idea. If your kitchen and/or bathroom is dry or
well-ventilated, consider drying clothes there to contain any
moisture. If those rooms are constantly damp, that won't be possible.
- Using a launderette may seem expensive, but remember that there are hidden costs to drying
things indoors at home: drying wet clothes in your house in winter will
increase your heating bills. That's not an opinion, it follows
directly from the laws of physics!
- Ventilate your home properly if you're routinely drying things indoors and consider buying
a dehumidifier if damp becomes a problem.
- If you're going to buy a tumble dryer, check out its
EnergyStar label (United States)
or an equivalent energy ecolabel (in other countries).
Remember that clothes dryers are just about the biggest energy-guzzlers in the home. If you buy the most
energy efficient machine possible, you'll save yourself a great deal of money over the years and help the environment too.
Photo: Energy-efficient drying: One reason tumble dryers cost so much to run is that most people "over-dry" their clothes. On older dryers, you select an arbitrary drying time and keep the dryer running until the clothes are dry. But that might mean the dryer is running much longer than it needs to. Modern dryers like this one, also made by Bosch, automatically stop when the clothes are dry. So instead of selecting a drying time, you select the type of clothes and how dry you want them—and let the dryer do the rest.
Find out more
On this website
There are hundreds of patents for all kinds of clothes dryers; here's a very small selection:
- US Patent 8,015,726: Automatic clothes dryer by James P. Carow, Whirlpool Corporation, September 13, 2011. A typical modern dryer with an improved ability to sense the flow of air through the drying compartment.
- US Patent 20090313848: Method and household appliance for drying wet laundry by Harald Moschütz, Bosch, December 24, 2009. A modern Bosch dryer with built-in temperature and dampness sensors.
- US Patent: 5,396,715: Microwave clothes dryer and method with fire protection by Richard D. Smith, Electric Power Research Institute, March 14, 1995. Microwaves can cook your dinner more quickly, so can they work the same magic on your wet laundry?
- US Patent: 4,490,923: Microwave clothes dryer by Perry W. Thomas, January 1, 1985. A very simple microwave dryer from the early 1980s.
If you liked this article...
You might like my new book, Atoms Under the Floorboards: The Surprising Science Hidden in Your Home, published worldwide by Bloomsbury.