Is it dry or wet? This is the most important question when choosing what to wear.
As the world continues to struggle with extreme heat waves, which are becoming more regular thanks to climate change, the clothes we wear play an important part in how we keep cool.
Researchers have discovered that with appropriate clothing it is possible to increase cooling by two degrees Celsius - which would save significant energy, money and reduce greenhouse gas emissions in the long term.
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So what should you wear to stay cool?
Color and design
When it comes to colors, most people wear white in the summer - because white reflects the sun's rays, instead of absorbing light like black.
However, this theory gets a little more complicated when we start talking about the thickness and fit of clothing, because heat doesn't just come from the sun - it also comes from our bodies.
When that heat from our bodies hits the white clothes, it is reflected back at us.
A 1980 study of why Bedouins—an indigenous, semi-nomadic people who inhabit desert regions of the Arabian Peninsula, the Middle East, and North Africa—wear black robes in the desert found that heat exposure was the same regardless of whether they were members of the tribe. wore black or white clothes.
How is that possible?
Black fabrics are better heat emitters - meaning they absorb heat leaving the body - so this can also play a role in cooling your body.
The Bedouin secret is to wear baggy black clothes, especially if it's windy. The loose black clothing warms the space between the fabric and the skin, creating an upward flow of air - like a chimney - and providing a cooling sensation.
"The amount of heat gained by a Bedouin exposed to the hot desert is the same regardless of whether he wears a black or white robe," the study states.
"The extra heat absorbed by the black cloak was lost before it reached the skin," it said.
Therefore, the cut of the clothes is actually more important than the color.
However, if you are going to wear tight clothing, then stick to white.
Textured fabrics - such as cheesecloth (seersucker) or dots (pique), fabrics often used in sports polo shirts - also help to separate the garment from the skin, instead of keeping it snug and tight.
Fabric is important
"Fabric choice is key," says stylist and fashion writer Heather Newberger.
"If you wear a baggy denim jumpsuit - you will feel more comfortable than your friend in a tight gauze or chiffon dress."
Lightweight woven materials such as cotton and silk tend to be more comfortable and fit better than knits.
This is especially important when it's humid - when it's dry and hot, absorbent alone may be enough as the sweat will be absorbed from the body and evaporate in the heat.
When it's humid and hot, the air around you is already saturated with water vapor, which means the sweat your clothes just absorbed has nowhere to go.
"In terms of clothing, it's better to wear a material that allows water vapor to pass through so it doesn't block the evaporation of sweat," says Rhett Allen, an associate professor of physics at Southeastern Louisiana University.
"Some of the newer materials used in sports do that. Cotton doesn't do very well with this."
All textiles trap infrared radiation emitted by the body to some extent, which helps keep us warm in cold weather - but it's not ideal for hot days, which is why wearing breathable clothing is important.
Uncoated cotton, linen, nylon and polyester are classified as breathable fabrics to some extent - meaning they allow sweat and heat to air through the material.
However, they differ from absorbent fabrics, which actively draw water from the body.
Cotton and polyester absorb and reflect most of the infrared radiation that hits them - almost 99 percent - which means they often appear white in infrared images.
These materials also transmit 30-40 percent of visible light.
This combination can cause the body to heat up faster than it would otherwise, according to researchers at the Massachusetts Institute of Technology (MIT).
This happens because incoming visible light can generate heat which then cannot escape as infrared radiation generated by the body.
But the body's other cooling mechanism - sweating - also plays a role.
Cotton absorbs moisture but does not dry quickly, so if you sweat a lot your clothes will remain wet, making them less comfortable.
Linen is widely worn because it has excellent breathability due to its large fibers, but it dries slowly like cotton.
Merino wool is a popular choice for outdoor enthusiasts because it is breathable and wicks away moisture without retaining odors.
Nylon and polyester are used in most sportswear because they wick away moisture and dry quickly - but retain odor.
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Research has also shown that nylon has greater moisture absorption and better absorption than polyester, but it dries more slowly.
Synthetic fibers like nylon and polyester can feel uncomfortable when they get wet, and one study suggested wearing clothes made from bamboo, which is a low conductor of heat and doesn't compromise comfort.
If you really want to cool off, strip down completely—as long as it's appropriate, of course, says George Havenit, professor of environmental physiology and ergonomics at Loughborough University in Leicestershire, UK.
Clothing protects your skin from burns, but it's better to be naked to keep cool.
The less clothing you wear, the more opportunities there are for evaporative heat exchange between the skin and the air.
Although, of course, protecting your skin from UV rays is a priority.
But there may be alternatives - new materials and fabrics that will help cool the body.
The new science
Sports companies such as Nike and Adidas are investing millions in designing smart fabrics, and scientists are investigating which fabrics are better at keeping the body cool.
MIT scientists have found that the key balance for more efficient heat dissipation from the body is a material that is opaque to visible light - it reflects and does not absorb sunlight - but transparent to infrared radiation - allowing heat to leave the body rather than being trapped between material and leather.
They found that using thinner nylon and polyester fibers - about one micrometer in diameter - and weaving them into yarn 30 micrometers thick could help people maintain a more comfortable temperature.
Scientists at the University of Maryland in the US have also developed synthetic fibers with coatings that actually change structure in response to external conditions, releasing more heat as the temperature rises.
Flexible yarn expands and contracts, changing the space between the fibers.
The larger gap allows the textile to breathe, allowing heat to radiate to keep the wearer cool.
Another group experimented with clothing woven with ribbons that flatten and bend to cool the body by more than two degrees Celsius.
When it's hot, the bands bend toward the air, dissipating heat from the body.
Other teams have investigated "phase change" materials that include capsules or fibers of material that melt as they are heated, helping to absorb excess heat.
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Getting wet
Perhaps the best way to dress to cool off in the heat is actually just to wear wet clothes, Allen points out.
Water needs heat energy to evaporate, and as it makes this transition from liquid to gas, it uses the heat coming from your body, cooling your skin and lowering your body temperature.
So it turns out that choosing what to wear to stay cool is a little more complicated than simply throwing on a white t-shirt.
But the right fabric, fit, and occasional water cooling when possible should help keep you cool when the mercury heats up, as well as save on air conditioning.
Watch the video: Cooldown Tricks
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