Cotton clothes are not your best friend when it comes to outdoor activities. They’re down there on the naughty list with jeans and knee-high, suede stilettos. This is not because some gear snob has decided that outdoorsy people are above cotton clothing. It’s actually to do with the science of staying warm.
You see, cotton is very good at keeping hold of water. Which is great, except when you’re in an environment where there’s a lot of water. For example on a miserable, wet day on the hills when it’s raining in all directions. Or on a humid day when you’re sweating buckets and getting wet from the inside out. The cotton soaks up water like a sponge.
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If you want to prove it, it’s actually quite a fun activity – particularly if you’re teaching kids. Get a bucket of water and two equivalent t-shirts: one synthetic and one cotton. Dunk both t-shirts in the water, then hang them out to dry next to each other. After an hour or so, come back to compare how damp the clothes are. The difference will be obvious.
The problem with this is twofold. Firstly, when the cotton is completely wet it’s no longer keeping you warm. This might be fine if it’s a scorching hot day, but less good most other times. Secondly, cotton has no wicking properties. This means it can’t move water (aka sweat) away from your skin into drier areas. You don’t get those big sweaty patches on a wicking baselayer.
If the weather is very cold or windy, you can get very cold by wearing cotton clothing. Particularly if you stop moving. Wet plus wind is well on the way to hypothermia. And sure, people climbed Everest in tweed jackets back in the day. But there are far more comfortable clothes nowadays, specifically designed for the outdoors.
