Is it dry or humid? This is the ultimate question when choosing what to wear.
In terms of clothes, it's better to have a material that allows water vapour to pass through so that it doesn't block the sweat evaporation – Rhett Allain
Material matters
"Your choice of fabric is crucial," says stylist and fashion writer Heather Newberger. "If you're wearing an oversized denim jumpsuit - you're going to feel way hotter than your friend in a more constricted outfit made of gauze or chiffon."
When it comes to fit, lightweight woven fabrics such as cotton and silk are usually better at hanging loosely than knits. This is especially important when it's humid – in dry heat, wicking alone may be enough because the sweat will be absorbed from your body and evaporate in the heat. When it's humid and hot, the air around you is already saturated with water vapour, meaning the sweat your clothes just soaked up doesn't have anywhere to go.
"In terms of clothes, it's better to have a material that allows water vapour to pass through so that it doesn't block the sweat evaporation." says Rhett Allain, associate professor of physics at Southeastern Louisiana University. "Some of the newer sports-based materials do this. Cotton does not do so great with this."
All textiles trap infrared radiation given off by the body to some degree, which helps to keep us warm in cold weather –but isn't ideal on a hot day and so wearing breathable clothes is important. Uncoated cotton, linen, nylon and polyester are all classed as breathable fabricsto some degree–meaning they allow sweat and heat to escape through the material. They're different to wicking fabrics, though, which actively pull water from your body.
Cotton and polyester absorb and reflect the majority of the infrared that hits them – nearly 99% – meaning they often appaer white in infrared images. But these materials also allow30-40%of visible light through. This combination can cause the body to warm up faster than it otherwise would, according to researchers at the Massachusetts Institute of Technology (MIT). This occurs because the incoming visible light can generate heat that can then not escape as infrared radiation generated by the body.
But the body's other cooling mechanism – sweating – also plays a role. Cotton absorbs moisture but it doesn't dry quickly, so if you're sweating a lot your clothes will stay wet, making them less comfortable. Linen is widely worn as it has excellent breathability due to its large fibres, but like cotton it is slow to dry. Merino wool has been a popular choice for outdoor enthusiasts as it's breathable and wicks moisture without retaining odour.
Nylon and polyester are used in most activewear as they wick moisture and dry quickly –but they retain odour. Research has also shown that nylon has a higher moisture absorption and better wicking capabilities than polyester, but is slower to dry. Synthetic fibres like nylon and polyester can feel uncomfortable when they get wet though, and one study suggested wearing clothing made of bamboo, which is a low conductor of heat, and doesn't compromise on comfort.
If you really want to keep cool, then strip off completely– as long as this is appropriate, of course,says George Havenith, a professor of environmental physiology and ergonomics at Loughborough University in Leicestershire, UK. Clothes protect your skin from burning, but being naked is better for keeping cool. The less clothing you wear, the more opportunity there is for an evaporative heat exchange between your 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 to cool the body.
